I think mids and putters might be the best idea. Maybe a fairway driver? Leopard, Panther or Spider, and top it off with an Aviar? Or Discraft equiv's. Or I've become a big fan of Latitude 64, nice discs.
I teach physical education and got a disc golf course set up donated to our playground with pro innova baskets/cement tees etc. The EDGE program "educational disc golf experience" recommends 150 gram Leopards, Sharks and Aviars for kids in dx plastic. They work well through Middle School or for anyone learning the game. The Edge Curriculum is great too :)
One of the favorites for the youngsters here in Clearwater is the Ultralight Raging Inferno DT made with Nike plastic.
My personal pick is always a 150 Stingray. One of the first discs I ever threw and still manipulate perfectly around the Disc Golf Course...learn on the light ones and move into the heavier later. A nice partner to the Stingray is the 150 gram Cobra which has a bead on the bottom for a touch more stability. We have the largest selection of Lightweight discs on the planet and make Custom Starter sets every day for new players of all ages. ClearwaterDiscGolfStore.com
I have an ultralight polecat and it has fantastic glide. It's also great as a catch disc for kids...which is probably a great way to teach a kid to throw.
I honestly don't think a kid of 5 needs more than a putter. I've found in teaching a group of newbies (an 11 year old and 3 middle aged adults) that they had the best success with a flippy putter. Drivers (even the light ones I gave them) didn't travel any further for them than the putter and were much more erratic.
If you want to get them some other light discs to grow into, I understand. I was seriously tempted to buy my 4 year old girl that pink bag and set of 20 light golf discs that someone was selling here....and to register her in the women's division at worlds this year. :-) Let the game come to them at a pace that doesn't frustrate or alienate them from the game... or you.
I think any player, young and old, weak or strong, should start off with just a putter. Once they start to max out the distance possibility of that putter (around 300') add a mid till they hit 400' and so on.
They'll definitely achieve more control over a shorter amount of time this way and that is the important part.
I agree with you about the putter start, but most people cannot throw a putter 300 ft. I throw an r-pro boss over 400 ft, but max out with my challenger at okay, well almost 300. But I think maybe at 150 ft with a putter they could benefit from an understable mid or even a driver.