Parents won’t do anything about flees

So I’ve got 5 cats at home. I’ve found flees on all of them and they have shown discomfort that I believe is due to the flees. I’ve told my parents about it but they are convinced there aren’t any flees because they haven’t seen any (I didn’t need to look too hard to find them btw). We even have some flea treatment solution we can put on them. I want to help them but I have no clue what I can even do.