Special Ranger with two pets?

Hi I am a new DM and will be gming a group of new players. Two of the players are a couple with two young girls who they want to be able to participate when we are playing. The one has played as a pet cheetah before and they allow her to do the capabilities of a cheetah listed in the players handbook. Well, our ranger wants to have two panthers as pets in order for both the girls to feel like they are participating. I dont want to give the ranger too much of an advantage with this. Would it be fair to say only one panther can attack per turn? But then would I have to worry about her having them flank all the time? Or is there a better way to handle this situation? I was originally planning on giving the party a Wondrous Figurine after they complete their first story arc, but it looks like both these girls will be playing at the same time now.