Phyrexian Obliterator is an absolute beast in a deck that can keep getting him back, so absolute ideal addition to Meren's forces. Also, doubling all death triggers with Drivnod, Carnage Dominus works very well.

I'd probably cut Thoughtcast for Crucible of Fire. The deck's draw potential is already very high and Cast isn't that great from the mid-game onwards, when it will be vastly outperformed by most other draw-resources in here anyways.

This looks like an absolute blast of a deck; well done! I've not come across many Izzet-dragon decks thus far, unless they featured Niv in some way :) I will definitely build this from my own collection, with maybe some slight modifications. I'd maybe consider Crucible of Fire to increase the strength of the tokens you generate (and of course all your other dragons too). I'd probably consider Steel Hellkite too, because its wipe is so powerful, especially against opposing token decks. In most tribal decks I build, I include Roaming Throne; not sure if that would make sense for this deck though, so just a thought.

This looks really nice and simple :) Considering that your strategy depends a lot on artifact-ETB that interacts with triggered abilities, why not include Panharmonicon to double your triggers? Also, Ojer Axonil, Deepest Might  ^Flip would seem to make a lot of sense, to increase your non-combat damage potential (some of it also caused by the triggers just mentioned).

I would also reconsider main-boarding Goblin Welder and Steel Hellkite. Welder because he allows you to exchange an artifact that's about to be destroyed by an opponent for an artifact that was already in your GY to return to play, which is insanely good. Hellkite because it's a very multi-functional wipe that will really come in handy against opponents that will try to go wide on you.

Gosh, there's a number of ways to go infinite with Selesnya-colors; especially in terms of tokens :)

If you need some examples, I would recommend one of the decks I enjoy playing most: Nature Strikes Back!

Thrakkus the Butcher seems to be card one would find useful in a dragon-themed deck :) Additional attacks through Hellkite Charger could also be fun when combined with Ureni. Savage Ventmaw can bring some additional mana to the table after attacking with it (also combined well with Charger's additional-attack ability). Hope your kid enjoys the deck!

Not sure you want to go this route, but considering you've added enchantments to increase your token generation, would it make sense to add more creatures in here that generate additional tokens too? Adeline, Resplendent Cathar, Brimaz, King of Oreskos, Emmara, Soul of the Accord, Mycoloth and Voice of Victory to name but a few effective ones. Also, the new Elspeth, Storm Slayer seems almost tailor-made for this deck. Granted, she's a bit expensive, but man can she bea powerhouse in here.