I would just say "I know you said you had a lot of diapers to use up but I'm wondering if you could use those ones at home and send bigger ones for daycare. I look after xxx amount of children and I unfortunately don't have the time to be constantly changing her to avoid leaks. If I don't change her every 20 minutes I am finding she is leaking through and it's making a bigger mess on the couch/bed/floor etc and it's just not sanitary."