Really any 150 class putter will be OK. I went with Magnets for my kids as they are a little more low profile than an Aviar, but they ended up getting Aviars and Wizards along the way and they both use their Aviars now. For my youth program I got an awesome deal from Gateway on 150ish Wizards. I was a little leery to give little kids on overstable disc like a Wizard, but it has turned out to be a non-issue. At any rate, I think having a putter and teaching them that they use a putter around the basket reinforces good disc selection so having a putter is important.
For a mid we used Sharks in the 130's and they turned out great. We tried a variety of mids, but the 130 Sharks were the most consistent in terms of the kids getting good throws. The discs that were terrible were the small-diameter mids (Spider, Skeeter, Kite.) The large-diameter discs responded to the low-power throws and glide out nicely. The small-diameter discs just seem to fall out.
For a driver to learn backhand drives I used LW Elements. Even though it is technically a mid, Elements have a big driver-like rim so you can get used to the power grip but you still get that low-power glidey flight from one. I have also used 150 class Stingrays and 160g Stratus for this and I would supposed a light Eclipse would be fine as well, but the Elements have a bigger rim and it was pretty easy to find them in really light weights. Again, large-diameter worked better than small-diameter. The old large-diameter drivers are slower and not so power dependent with good glide, so they have worked better than the 150 class Leopards and Cheetahs that I thought were going to be the best discs for this. They just don't make very many of the large-diameter drivers any more.
We so far have run 41 kids through our youth program, and none of them could forehand at all. I was pretty surprised by that, but we had a lot more success with backhands than forehands. I've seen the same videos with tiny kids crushing shots with a baseball slinging motion, so obviously it works really well when they can do it. We just have not had any kids that can do it. I'm not sure what discs would work best for teaching this. I'm assuming your basic small-diameter driver like an Eagle or something like that is fine, but I don't really know.
So for me the best four disc bag for young kids (we are working with the 7-10 year old range) would be:
1. Putter: 150 class Magnet or Aviar P & A
2. Mid: LW Shark.
3. Driver: LW Element.
4. Some small-diameter driver to work on forehands.