What Causes Impetigo With Small Blisters In A Child?

Daughter 7 years old started with impetigo - small but many with blisters (bullous?) on the buttocks and teh 10 months old small sister got it as well but on different parts of the body (legs, arms, buttocks face and back). Not very large. Me and my husband got some as well but only a few (5-6) on the buttocks, legs, back, face. We were treated with heracillin (flucloxacillin) for 12 days. The small one didn?t get any effect at all, my husband got rid of his and the last day the 7 year old got rid of hers. I had 2 left. We treated the blisters with bactroban on the skin and the small one got better, allmost free from them and me as well. 1 week later the 7 year old got them again on the but and it was starting all over again on the small one and on us.... They have taken test on the 7-year olds blisters but say that there is to little liquid to get a result. Nasopharynx i s taken on us adults but nothing there. We clean thourougly every day, wash all clothes, towels and linnen every day. shower every day and wash hands often and thouroughly. The 10 month old is having a surgery in 3 1/2 weekns- I guess not a good idea with staff infection?!! What can we do to get rid of it? After 8 weeks of cleaning and washing and not being social to avoid giving it to others we?re totally exhausted. Please help.Cecilia