Personally I would give them more than 4 weeks because it can be hard for those who do need long hours to find a daycare that offers them those hours. I would say around 6-8 weeks. I guess that means I would tell them pretty much as of next week since you already know what day you want it to take effect in May.

I hope it goes smoothly and that you enjoy the extra breathing room of shorter hours. My hat goes off to anyone who does offer more than 10 hours because that is a looooooong day. I think enjoying the time with your own kids is great