Did you not have this discussion when you interviewed? I would meet with them and let them know that these are full time spots (or minimum 4 day a week spots) or whatever you thought you agreed to. I would have them pay a set weekly fee (regardless when they come) if they have an erratic schedule so that you can pay your bills and have a steady income. If you need full time from your under 2's then say as much and start interviewing.