Job acceptance regret

So this week I got the opportunity for a second job. It's part time, one of two days a week, at a daycare as an assistant. I'm already a nanny 3 days a week. I took the job without really thinking about it and my current employers referred me here because they knew the daycare needed extra help. I'm supposed to start tomorrow but I keep feeling regret that I agreed to this. I didn't think about it at all. My health isn't the greatest and I know I need to focus on that as much as I can. How do I go about telling the daycare I'm not able to do this? It isn't a big daycare. One main lady, an assistant, and then me who would be the fill in for the main assistant.