If they are not adequately supervising the children--that is, if their level of supervision is less than a reasonable daycare would do--then they may be liable for any injuries. But it's not clear that it's worth suing economically: as distressing as what has happened is, you do not indicate that your son has suffered permanent (or at least long-lasting) significant or serious disability or life impairment, or large out of pocket medical bills. But when you sue, you can only get compensation equivalent to your actual costs, losses, or the seriousness of the physical injury. Based on what you write, you could spend more on the lawsuit than you would get back.