I have taught children's whistle for years - depends on the age of the child as to what is a good whistle to start on and whether they are learning to play on their own or with a group. Under 7's I tend to use a g as the whistle is smaller and easier for them to get the notes properly and if they are playing with older ones I just transpose the music for the D whistle to fit with the G. For real little ones I start them on an f or start them with an ocarina until they get used to moving their fingers about to get a tune.

If they are just trying out and not really very serious then sweetones, feadogs or generations - check out C&F as advised above and they have advice as to how to get better tuning out of a cheap whistle - I use a bit of blutac

personally I like the susato and so does my 9 year old daughter - easy to play and get the higher octaves aswell as easy to tune but as the others have mentioned more expensive.

A friend of mine only ever plays generation whistles so it is a matter of preference.