
I'd not use a crate because you need to be able to get in there, if necessary. I realise this is too much for your small breed b itch but we had a 4 X 4 whelping box, made of plywood, with a wood floor and 2 ft sides ( may have been higher, I forget) and that section had pig rails round 3 sides with a low front for mum to access the box but the puppies couldn't get out at first. Once the first three weeks were over and the puppies were up and moving around, we moved the box into the kitchen and added a second 4 X 4 section, removing the low front of the original section. The second section had a hinged door for mum to come and go. We had a gallows up the original end, for a heat lamp. My husband made this and it was scrubbed clean and disassembled for storing after the last puppies went home. The two we usually kept (we only bred for our next generation) then went into a 'puppy pen' which we had under where the stairs went up, in the kitchen. That had a raised bench in it.
You might use a kids paddling pool, lined of course.