Because children have smaller hands a higher key whistle might well help, in which case buy two Fs and work with your child. Make sure you try them out in the shop and get ones with decent tone that arent prone to squeaking.
A decent Generation is a good choice for a child because they are relatively easy to learn to blow properly. Id definitely work on real tunes, even nursery rhymes.
I often start with The Leitrim Fancy with beginners of any age because of its step-wise shapes in the A music (and because it brings in C natural cross fingering from the beginning and is good for explaining cuts and rolls on - but all that wont quite concern you yet!) Choose tunes that start in the left hand and dont go over the break too much.
It is also a very good idea to explain how things work to children - in a way they can grasp at their particular level, of course. If your child has problem placing his fingers on the holes and has to keep looking down, dont worry, quite quickly he ll feel for placing the fingers, but it would probably help if you explain to him why her fingers must cover and seal the holes and come off/go on in order.
You can show them that yourself (contrast doing it right with doing it wrong so he can see the differences).
Try things like getting them to finger the holes while you stop up the top end and blow in the bottom so they can feel any leaks.
Also get them to blow while you finger a tune and vice versa.... anything like that that can be made fun.
Show them and try to get them to explain to you about "higher" and "lower" pitches and longer/shorter tubes and how the tone holes lengthen or shorten the working part of the tube. A leaky finger fails to lengthen the tube properly.... If they understand the elements of why it works one way and not the other, they will be more likely to enjoy the challenge of succeeding and be able to target success for themselves.
All this takes much more complexity to explain than it does to actually do! Think out ploys for yourself and try them. Dont do too much at one go; choose a time when the child is enthusiastic, run with it while it flows, be prepared to drop it and return if it palls.
I wouldnt emphasise scales to such a little children (3-8) - too likely to bore them unless they have a very studious inclination. Get those in the context of well chosen melodies.
Ultimately, the key is to get them and keep them enjoying it. If they are interested and engaged, they are never too young, and even if they dont persist with it, it will have laid good foundations.74