I do not recommend using on a pet products that are not approved to be used on animals and which were not safety tested, especially on young animals. If you wish to wash your pet the best time to do so is before the flea treatments but please use a proper dog shampoo.
The skin needs some time to reach its normal equilibrium in pH and fatty substances. The spot on products are carried on the skin surface by these fat substances so if you were to wash your dog after the treatment a large amount of the flea solution would be washed by the detergents in the shampoo.
After the treatment it is advised to delay for as long as possible the bath of your pet, if possible until before a new treatment is due.