I've got a problem with my sandpit?

Good idea !

But go with 2 to 3 feet of gravel !

Will need to get a longer pin to

insert deeper in to the ground

below gravel so it will be stable !

Depth of gravel will give you a dry well !

I would take a 2x6 or what ever your back board height is and cut board 3/4 inch's longer. Take board and cut diagonal making two odd shaped triangles. With the back end (high end of th board) of boards spaced just barely wider than back board of pit. Attach a furing strip or rip down a piece from left over board. If you nail all 3 Pisces together , should look something like a ramp kinda. Now take ya some thick plastic like visqween and cover board with staple gun and pulling tight. Mark where your cover needs a hole for shoe bar with a marker and add duct tape 3/4 inch's past hole both ways overlapping. Pop ya hole through tape and there you got ya a removable cover cheap easy to replace plastic , and frame wont bend break and dont cost ya no then but a 6pack of beers time on a sat. afternoon.

Thumbs up for dude above. Move to a location that drains well. But before doing that, do you have washed course sand? If not, try it. Sand in well drained areas percs water and dries fast. The fact you are having trouble seems to indicate you have no perc and adding a base is not going to change that.

Getting back to washed sand. Are you sure? Some sands that get used are full of clay fines. They don't perc and take forever to dry.

could be you have your pit in a low aera . if you do try moving them to a higher spot on the property where the water can drain away .

Sounds like you have some big cats in your neighborhood.