We have a 4 year old male beagle who in the past month has tried to bite 2 neighbor kids when he was outside?

We have a 4 year old male beagle who is extremely loveable! In the past 2 months he snipped at a neighbor girl that he knows when she was on her bike. At first I thought it was the movement of the bike that he didn't like. Today the same little girl walked into our yard to pet him and he snipped at her again. My husband was able to grab him otherwise he would have bitten her. We don't understand what is causing him to do this. He has never wanted to bite anyone. He is such a loveable dog! Please give me any answers that might be causing him behavior. He was on a leash in our front yard.