If a bathtub is big enough to be under there, can a human fit? Or do you mean you can hear them under the bathtub on the floor above? Kittens are born pretty much deaf and dumb, they can't get around well and it will be weeks before they venture out. Make sure mama is getting plenty to eat and drink, NO cows milk products, cats are pretty much lactose intolerant to cows milk. Goats milk and water are fine, as are small amounts of heavy cow's cream which has almost no lactose. They will venture out when they are ready but would need to be socialized to humans. Maybe mama will decide to move them if you could make her a nice, secluded place somewhere else, like in a shed. For now, I would just let them be. I hope you can find a way to get them!
Pregnant cats should ideally be kept inside towards the end of their pregnancies for this very reason. I doubt she’ll move them soon, but they will venture out at around 6 weeks. Do you plan to spay your cat so she stops breeding?