My kids are almost exactly the same ages as yours and they all bathe together. They have no issues with it, and neither do I or DH. Until they want to be alone, they will bathe together! Similar to what a pp said, I also shower/bath with the kids, as does DH, and I have no issues with that either. DD and DS1 also bathe with their older cousin (a girl if it makes any difference!) who is 8 without issues too.