If they want to do it, they might as well do it the right way and with supervision.
A book of fascinating but slightly dangerous experiments that kids would love to do. The book explains the tests, the danger involved and give exact measures to protect the kids and prevent the danger from happening.
Private and small swimming pools are unattended many times, and that can be dangerous especially for little children.
When sensors indicate that the pool has not been used for a while, a net is automatically stretched across it so that anyone who might trip into it will be suspended unharmed above the water.