Sheikh, what is the best age to teach your children about the deen?
There is no particular age to start. This can begin from as early as few months old.
Mention Allah in all what you do and attribute all good things to Him. Stick to teaching them surahs and encouraging them in imitating you pray when they are very small.
Teach the daily Sunnahs of eating, sleeping etc.
Instruct them to pray when they are 7.