They do have skin in the game. As shown in many studies, people generally are good. Therefore, most daycare workers would prefer if the children they worked with turned out good rather than bad.

While daycare workers may be underpaid, in many countries they work requires at least some education. They would also accumulate experience in proper child care the more years of experience they have in the profession. I.e. it is not a given that mothers are better than professionals in raising their own kids.