Requirements

1. 1. Waterproof.
2. 2. Multi-part for it's easy to handle. The current cover is a single piece which is nigh impossible for a single person to handle.
3. 3. Windproof. Shouldn't blow off in a wind. This is a fairly minor point as the sandpit is fairly sheltered, but implies that it should have a little bit of weight to it at least.
4. 4. Simple. Don't have that much time to spend on it.

Design

Thinking of a three-part lid, with the middle part raised so it can lip over the two end pieces. Using 16x16mm beading for lips on the end pieces, with 16x32, or 2 x 16 x16mm beading for the middle piece to raise it. It thus won't be 100% waterproof, but the leakage should be pretty trivial given the existing lip on the sandpit.