The Dambusters Play Set arrives in a plastic tub. After you've inflated the paddling pool, your first task is to build the dam. The pool has a rigid plastic frame supporting the opening you're trying to fill. The sand isn't provided, but the binder is, in sachets to limit the amount of "cement" you can use. The lid from the plastic tub has a convenient convex shape to it and serves as a mould for beginner dambuilders. When the sand has set, the shape-forming parts are removed and the pool should be ready to fill.
Now the kids (and grown-ups) get to take it in turns to play dambuster. The launcher has wings to give it the approximate appearance of a Lancaster but it's actually a kind of catapult that sits on a tripod which you can carefully adjust. The stores themselves ("bombs") are plastic cylinders not dissimilar to film cannisters which need to be filled with baking soda and vinegar. Internally, the canister has a divide to delay the mixing of the ingredients - loading the cannisters is notoriously fiddly.
The launcher, as you can probably guess, imparts spin to the bomb and fires it. This is achieved using elastic bands, which need to be wound the correct number of turns (to be determined by you).
Oh, there are plenty of things to go wrong - too many bounces and you may bounce right over, too few and it might not reach. The bombs themselves will almost certainly detonate prematurely. Do you impart topspin or backspin? What is the best angle of first impact? You may need to add ballast to your bombs to help them sink, even if this means compromising on payload. You will hopefully find that a small blast in the right place can do far more damage than many large near-misses.
Inevitably, the dam will eventually break - but will it be by your bomb, an opponent's, or the misuse of the catapult in frustration? Either way, the game ends with destruction and the flooding of your garden, and yet the toy can still claim to have enormous historic and scientific educational value. Bravo!
Some more info:
Originally I wanted the canisters to have a lid which popped from concave to convex when at a certain depth, but even the slightest amount of pre-mixing quickly overpowers the effect of external pressure. We don't want to overengineer the bombs, they should be simple and almost disposable, so perhaps the best solution is a 'canister within a canister', where the baking soda is in the inner container, with a calibrated leak. At some point the inner container will burst and suddenly mix the remaining ingredients.
Choosing a non-toxic binder might be difficult, so it may be that this part has to be supervised. But it would add a lot of replayability if different dam shapes could be experimented with.
For imparting spin, it may be easiest to have a surface along the length of the catapult which the cylinder rolls against. This is harder to adjust but we're probably dealing with too many variables anyway.