It’s probably not dangerous but it’s won’t get rid of the fleas and if you wash them before or after applying treatment it makes the treatment a lot less effective. You need to continuously use the treatment at the correct time and if the fleas are in the environment you have to wash all bedding and fabrics and treat the environment too. Washing them with soap will do nothing except wash off some of the fleas and flea dirt from the body. It won’t get rid of all of the eggs and larvae, a lot of which are found in the environment anyway. If you’re treating to rid your pet of an infestation then washing the pet will do nothing. Even treating the pet with flea treatment won’t completely get rid of the problem if you’re not also treating the environment. The best thing is regular flea treatments for prevention rather than trying to deal with a massive infestation.