My beautiful neice is a tall 3 year old (she will be 4 in April, and I am not sure of her exact measurements, but she is tall and slim.) She is very advanced verbally and also incredibly well behaved. My SIL has got exactly the same concerns that you have, and we are all probably at fault of treating her like more of a 7 year old.
So I don't have any real suggestions to you, but I shall mention this post to my SIL and see if she has any great words of wisdom!
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