For kids that young, I’d lean towards PLA. It’s food-safe (when printed with the right filament), doesn’t emit strong fumes, and is easier to print. It’s fairly rigid and dimensionally stable, which helps with fitting parts together. Plus, PLA can be sanded and painted easily for a nice tactile finish. ABS is tougher but emits fumes during printing, which might be a concern. Nylon is durable and flexible but can absorb moisture and warp, making it tricky for tight fits and small runs. If you want more durability, consider PETG as a middle ground, it’s tougher than PLA but less tricky than ABS or Nylon.