My husband seems to not see this as inappropriate;

He came home from work and our kids ran to hug him as usual… and as usual, they started playing… my husband started tickling both of our kids(boy and girl)… I noticed my husband tickle our daughter’s bum and I firmly removed his hand. To him he was just tickling her and didn’t realise how inappropriate that might have been. In front of my husband, our daughter said to me “I didnt like it when dad touched my bum”. I gently cautioned my husband and I told him not to tickle her that way again. He was shocked and didn’t understand how that is seen as inappropriate! He is very playful with the kids and does not understand what he did wrong. Am I over reacting here?