A big round of applause, mate. You've done a lovely job on it and your kid looks really happy. Great that you involved him in the build and hopefully he'll persevere with the playing.
I'd locate the strap buttons in a similar place as on a real Warlock. It's actually been hard to find a picture of the back of one, but I found this, which should provide enough information for you:
Just don't put them too near an edge. I'd also suggest using a dab of PVA or Titebond glue on the screws as it helps keep them in place. Because of the strap button locations, I'd suggest swapping the standard buttons to straplocks, as there's going to be a bit of a twist on the strap when it's on the buttons, and you don't want to see it falling off.