How much notice for super informal job?

How much notice would you give a family you only nanny for twice a week? I’ve been working with this family (no contract very informal, literally discuss the next weeks schedule on fridays before I leave for the day) since last month but have recently started a second job that wants me for more hours than originally discussed. Currently on the days I nanny for both families it’s super hectic (I work for the 2x a week family for only 3 hours, then have to commute to the other job) and my total commute going back and forth from my house to each job on those days ends up being 2 1/2 to 3 hours (public transit). I want to let this family know that I need to leave because I need the security of more hours that the other job offers but don’t know how to go about it. I also love the kid and he’s shy around most people so I’m hesitating to part ways because I don’t wanna hurt him :(