I'mCustomerand I will be glad to help with your question. Frontline products are approved for use on pregnant and nursing female dogs. The small amount they may contact from touching her neck will not harm them. In addition, it is fine to deworm Katie with Nemex too. I would suggest getting a flea comb to manually remove fleas from the puppies:
You may also be interested to know that Frontline spray is approved for puppies at least 2 days of age:
This is Merial's Frontline frequently asked questions page where you can verify recommendations for use of Frontline & Frontline Spray:
I hope that the information I provided has been helpful. Please remember to select REPLY TO EXPERT if you have more questions or would like additional information. It is my goal to provide you with the most complete information possible prior to you leaving a feedback rating. If you received all the information you needed, then kindly submit a rating.