When someone does us a kindness, what words and attitude does the sunnah teach in return?

Returning kindness with thanks and prayer is the character of a believer, at the very least praying for good for that person. The Prophet ﷺ said: “Whoever does not thank people has not thanked Allah” (Sunan Abu Dawud 4811). Teach your children to say thank you and pray for anyone who helps them. If you can, return it with an equal or better kindness. Avoid accepting a favour without appreciating it, because that weakens affection. Start practising it among family members first, because they most deserve your appreciation. See everyday adab.