We have done BBB many times (10+) and always do in at the beginning of our trip so that she can wear her hairdo for a few days. I let dd7 swim & typically she just doesn't go underwater. In Sept it was boiling hot so after the first day we scrapped that and just let her swim. That time she had the fairytale princess do so all we did was take out her crown & believe it or not the bun stayed in fine.
It is all for fun, so if they want to keep it in let them and if not let them swim, they are only little once, so enjoy it!