If your kid wants to write their own side-scrolling shooter game, they probably need to learn to program.
My 9-yr old is learning to code using Phrogram (a revision of KPL - Kid's Programming Language), which is based on the Microsoft Visual Basic IDE model, and uses a simple BASIC-like procedural language that has sprites and intersection detection - so it's easy to do games.
Phrogram runs on top of .Net, so it's Windows-only, but it has a nice tutorial, and a ton of example programs. It's also free. Phrogram runs pretty slow, so you need a fast machine.
So far, we spend about 15 minutes a night going over loops, conditionals, and methods. Here's some sample code:
For J = 1 To NumberOfSticks If I <> J Then If Sticks[I].Intersects(Sticks[J]) Then HasIntersections = True Break End If End If Next
BTW, Let me know if I'm missing something great out there - the modern day equivalent of "Bill Budge's Pinball Construction Kit" game...