I will enforce nap time. Up till the age of 5 years old I believe they still need a nap. They can go without nap but they will get cranky in the evening and may even fall asleep during dinner time. When they are six I won't enforce it unless they are tired because by 7 years old they start primary school and will likely drop off their nap.
For your 2 and 3 year old kids you defintely need to make them nap. Perhaps being together makes it harder for them to nap as they will want to play. Have you tried separating them during nap time?
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Domestic diva of 3 rugrats