In decathlon today and found this tiny paddling pool. It won't need much filling and folds into small pack size. A couple of buckets of warm water (or even the bag it comes in) - sit a grand kid in with a beaker.
We used a tiny blow up paddling pool, about 2ft diameter (bought from Woolworths for £1.99 I think.)
It fitted nicely in the smallest of shower trays, so we could use the shower to fill it, then just let the air out, and the water ran down the shower drain.