Just speculating, I have never done anything like this. I bet you could pull this off with a container (Like a clean IBC tote) and a well water pump. Have the

Just speculating, I have never done anything like this. I bet you could pull this off with a container (Like a clean IBC tote) and a well water pump. Have the drain go back to the tote. Setup a sink off of the well water line. It will keep the lines pressurized and usable. Then have the sink drain back into the tote. Would want to treat the water like pool water to keep it from getting gross. a little filter and some shock/chlorine. It is not a bath tub, but would still let them enjoy a sink. The pump might be loud if you but a cheaper one which could be an issue for the child. Might be able to rig a sump in the tote instead (cheaper) but you would need to figure out how you want turn it on since it would always be submerged in water - the normal switch would always be on. Good luck - would be interested in knowing what solution you come up with. Edit - After looking around a bit, you can pick up a grey water recycling system. They have the pumps, filters, container all bundled into a kit. This seems like the easiest (but not exactly cheapest) way to go. While speculating above I was basically re-inventing one of these. Also it looks like you can scale this down to an RV sized system or up to whole house.