@Kimberly1 Your listing has a max occupancy of 3, but it looks like you have an extra person fee for a 3rd guest. This group was probably trying to evade the fee by pretending to be a party of two. One way to approach this would be to feign benefit of the doubt, and sent them a booking modification request to make sure the correct number of guests are registered and paid for. If mom accepts the change, the payment should be updated accordingly.
If she ignores or declines the request, you have to decide whether it's worth pursuing a payment dispute in Resolutions or letting it go and leaving an appropriately negative review.
If you don't care about the money, you can simply acknowledge the extra (presumptive) child and ask if they need an extra towel/set of linens. But if you felt like just letting it go, I guess you wouldn't have been posting here...