As long as the puppy is 7 weeks of age or older the Seresto collar is safe for the puppies.
Make sure the Seresto Collar is tight enough on your dogs neck so it's not lose enough for them to try and grab with their mouth but just make sure it's not too tight.
If the dog or puppy is able to chew on the Seresto Collar then it means that it's too lose and needs tightened up a bit.