Hi Little Miss Sunshine,
I'd add my welcome too and impart what advice I have read recently. Some people say grow stuff that is expensive to buy or hard to get...... well I can see the sense in that but when it comes to kids, growing what they are likely to eat is probably more common sense.
Start off with some staples such as spuds, peas, carrots and lettuce/salad leaves. People rave about beetroot, which grows well and can be used in all sorts of ways.
I love the idea of measuring pumpkins as they grow!
To keep them interested, some quick cropping veg like kohl rabi will let them see the from plot to plate transition early enough in the summer that they don't lose interest.
Good luck.