So I pulled my tarpaulin out of the shed and put tape over the holes.
Then carefully (with the help of the Mother In Law), taped the edges together, leaving a hole for the hose to go in. Looked good enough, so I shoved the hose in and started to fill it. Turns out that actually it's not so easy to seal it! No matter how much tape I put on, leaks sprung everywhere!
It wasn't really a huge issue until the water started running into the "digging area" ("I'm going into the digging area" explained The Boy, "We don't have a digging area!" I said, "We do now!") and then The Eldest filled up a watering can with muddy water from the hole and helpfully poured it all over tarpaulin... Oh well!
The Baby wasn't too keen on it initially, but eventually had a bit of a play on it.
The best fun seemed to come from all the water that ended up in the "digging area"...
I will give it another go, perhaps with a different tarpaulin, or wider gaffer tape and I'll let you know if I have more success!