Unmanageable Tantrums

I'm am pregnant with my first but have been watching an 18 month old boy for the last few months. He is great and we have little things here and there but it works for us. I was asked by another family to watch their 18 month old boy for a few weeks while they find a provider. He throws tantrums like crazy. I am used to one on one care with families I'm close with and normally manage this differently than I can. I have no way of reasoning with this boy or having any sort of consequence for his actions. I feel bad because I understand a lot of it isn't his fault. He screams and kicks and head butts any time I'm out of sight and being pregnant today I have had intestinal issues and needed to use the bathroom much more. But every time I do is a new screaming kicking match. It's only eleven am and he has had ten meltdowns. He's mad that he can't eat in the living room, that I changed his diaper, and that the other little boy had a toy he wanted first. He is upsetting my other little boy and there is no way he can take his nap. This is the second day of this and I can't help but feel that my responsibility is to the first family I care for. Is it wrong of me to ask the other parents to come get their child? What would I even say? I feel terrible but my normal boy is not getting the normal care and I can see it in his actions and tiredness overall.