What Would You Do?

So I watch two little boys during the day with my 2 month old. I started back with them when I was 4 weeks postpartum and it was pretty exhausting but I managed. The youngest one is two and he's going through this horrible tantrum phase where he hits, yells, bites, throws things, Etc. Well today I was buckling him into his car seat during his meltdown and had my 2 month old in her carrier and he kicked her so hard in the back of the head that he gave her a mild concussion, we just left the hospital. We need the extra income but I just don't want to bring her there anymore and I want to be with her during the day. I so wish I could be a stay-at-home mom, but it's just not in the budget. Anyone have any idea what I can do to make sure this doesn't happen again? I feel like a mom failure. I do want to say that he felt really bad and he didn't mean to hurt her he just doesn't know how to express his emotions oh, he's not a bad kid