Personally I bathed my kids nearly every day since they were born. They played outside a lot, and tended to always be dirty. Especially DD. Always in a dress, and filthy.
Son was pretty much able to do the whole wash himself thing from about 4 on. He was good with showers too. Never got weird about it getting his eyes. I kept his hair pretty much buzz cut then, so if he didn't do a great job one day washing it, I would make sure he did the next.
DD on the other hand sounds a lot like yours, loved the tub, wanted to swim in it. She also had 10 tons of hair, and really wasn't good at washing it or brushing it on her own until she was about 8.
I did bath them together a lot, and probably longer than most parents would, hell it was just easier that way. Fill the tub throw em both in, let em soak for 10 minutes to loosten the grime, scrub em both down good, rinse em, and let em play for another 10 minutes. Of course it was all part of my military presicion bedtime routine, i was very militant about bedtime, and sleep, still am. Winter was the best when they couldn't tell time though....I could get them to bed at 6:30 or 7 a lot. That left a whole evening for me. but I digress.
I would not leave them alone in the tub, but would bring a load of wash into the bathroom to fold, or pluck my brows, or busy myself but keep an eye on them.