How far does one go in maintaining convictions and standards for the family, possibly offending other families in the assembly?
The question uses the word "offend" which calls to mind Romans 14 and 1 Corinthians 8. In these passages, love for my "weak" Christian brothers and sisters imposes limits on my liberty. However, in the context, the "weak" brother is likely a new and immature believer who is bringing in significant religious or cultural baggage from his pre-Christian life. This is not likely descriptive of people commonly encountered in the fellowship of the questioner.
The feelings and spiritual progress of our fellow believers are important and we bear a measure of responsibility for them. But we bear much more responsibility for our families and the way that our own homes are administered. If we try to raise our children for the Lord, we will likely find that others disapprove of our decisions at times. Sometimes well be considered too strict and sometimes not strict enough. As young parents, we should be open to the counsel of others (Prov 11:14), but need to understand that we are ultimately responsible to guide our homes in "faith and love and holiness, with self-control" (1Tim 2:15 ESV).
If our children are aware that we are setting a rule for them that is different from the practice of another family, we should carefully consider the timing in giving them our explanation. We do not want to embarrass other families; we want to communicate the need to respect and not insult our fellow Christians.
Joshua said, "As for me and my house, we will serve the LORD" (Josh 24:15). Humbly and politely (Prov 15:1), we must maintain the standards in our home that we think will be the greatest spiritual blessing for our children in their future, fearing God above men. Notice, it is the virtuous woman who "fears the Lord," (Prov 31:30) whose "children rise up and call her blessed" (Prov 31:28). And it is the "just man" that "walketh in his integrity" whose "children are blessed after him" (Prov 20:7).
Is it right to speak to our children as if saved; for example, asking them what they think would please the Lord in a certain situation?
Teaching our children what pleases the Lord is essential and is not the same as speaking to them "as if saved." While we never want to convey to our children that they are somehow already "safe" just because they are in a Christian home, we do want them to reap to the fullest the advantages of learning Gods truth from infancy. Recall that Timothy learned the Scriptures from Lois and Eunice (2Tim 1:5), "which are able to make thee wise unto salvation through faith which is in Christ Jesus" (2Tim 3:15). By teaching him the Scriptures, they taught him to fear the Lord, a phrase that appears nine times in Deuteronomy, including Deuteronomy 31:13; that the "children may learn to fear the Lord." Part of "fearing the Lord" is considering what the Lord thinks of ones actions. Notice then the New Testament teaching to fathers regarding children: "bring them up in the discipline and instruction of the Lord" (Eph 6:4 ESV). Always asking the child "what would God think about this" is not likely the wisest path. But developing a mind-set in the child that they are responsible before the Lord is healthy, and the question of what pleases the Lord may help to instill in the child that this is paramount in our lives. Occasionally, the principle of Romans 8:8 should be interwoven: "They that are in the flesh cannot please God." This is not contradictory to the above, but will further help the child to understand sin and his need of God.
Is it Scriptural to teach our unsaved children to pray and give thanks for their food?
Assuming the questioner has in mind the children saying prayers themselves and giving thanks with their own lips, it is not Scriptural in the sense of having a clear Scripture mandate, nor is it unscripturual in the sense of contravening Gods Word.
Based on Ephesians 6:4 and 2 Timothy 3:15, it is obviously Scriptural to teach our children the Bible. This embraces more than the basics of the gospel; our responsibility is broader. There are at least three reasons for this. First, the Scriptures make one wise unto salvation (2Tim 3:15). Second, living by Biblical principles is a healthy guideline for life (Prov 13:15 and Eph 6:2-3). Third, ponder Proverbs 22:6. "Train up a child in the way he should go: and when he is old, he will not depart from it." As a Bible teacher of a previous generation taught, "bring your children up in the faith." This tells us to teach them the principles of Scripture with the expectation of their future blessing, although we know their salvation is not guaranteed.
The Bible teaches us to be thankful for food (John 6:11; 1Tim 4:3-5). The Psalms are full of calls to "give thanks," because gratefulness is a beautiful characteristic for a needy and dependent humanity. By contrast, ungratefulness is highlighted as a description of a rebellious world in Romans 1:21 and the perilous times of the last days in 2 Timothy 3:2. Obviously, we want to cultivate thankfulness and dependence on God in our children. Our generous Father in heaven "maketh His sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust" (Matt 5:45), so both believers and unbelievers should be thankful to God for good. Why wait until they are saved, when they can have the advantage of making it the habit of their life from childhood?
The same goes for prayer. Nowhere does the Bible say that unbelievers "cannot pray for themselves." The key issue is the attitude that generates the prayer (recall the Pharisee and publican in Luke 18). At the beginning of Acts 10, Cornelius was clearly not saved (10:44 and 11:14). But it is said of Cornelius that he "prayed to God alway" (Acts 10:2) and that his prayers came up "for a memorial before God" (Acts 10:4). Also, remember that the Lord taught the need to become like a child (Matt 18:1-4). One of the features of a child is childlike trust. If the principles of the power of prayer, and dependence, and the ability of God to answer prayer can all be instilled in ones youth, are not children much better off when they ultimately do embrace the gospel? Communicating with God will be an assumed part of their life.
The Bible does not command us to get our children to do these two things on their own. We must example it to them, but they are advantaged if encouraged to pray in their own words.