We also use the Revolution.
It is dosed based on weight which is closer to our babies sizes. They have a 1-5 lb and 5-10 lb dosage. some of the meds cover too broad of weight (i.e. 1-30 lbs.) and those are way too strong for our little ones. We have never had a problem with the Revolution. It is topical on the back of their necks and you can bathe them after 24 hours of application although i always wait a week afterwards just to make sure.