J.J.
With your daughter she is going to have all the same parts as you so I don't think it is a problem unless she is uncomfortable with it. With your son they say that you should stop when they start noticing the difference between your parts and his. My son is two and he already notices a differences but he loves me to play with him in the bath tub. Children usually start having memories (the earliest memory of your childhood) from the ages of 3 to 5 so I am starting to slow down bathing with my son now so by 3 he is used to being in the tub by himself and it gives me a break while he is playing in there by himself to do stuff around the house. I put enough water in there so he can lay on his tummy and keep his head out of the water and I check on him often.