Clam shell style sandpit/paddling pool

[DontBuyANewMumCashmere]

Has anyone managed to 'seal' a clam shell style sandpit?
We take the top off during the day and use it as a paddling pool.
Every night it goes back on and we use those little re-usable clips to fasten it on but there is always a shocking amount of crap in it after a few weeks. (We also replace the sand every summer.)

Can anyone recommend a way to insect/rain proof it?
If we lay a sheet of tarpaulin, or a ring of silicone or foam or something?!

I'm not at all anal about cleaning/garden/housework there's just something about watching my DC play in a shit tonne of fetid water and insect dung that turns my stomach!

Thanks for any advice.

We used to put a large sheet of tarpaulin on the ground under the sandpit. When the kids had finished playing we would put the lid back on and then bring the tarp up over the sandpit (like wrapping a present) and then fasten it all in place with DIY clamps, such as these:
www.diy.com/departments/spring-clamp-set-of-6/1801966_BQ.prd

If you are neither filtering the water nor treating it with chlorine or salt then it's going to get fetid after a few days however well you seal it up!

I think op is emptying the water daily but the lid isn't secure so insects and rainwater are getting in.