Cannot watch my grandson any longer :(?

I'm a 50 YO mother of a 29YO son, 25YO son and a 4YO little girl (adopted 2 years ago). I'm also the grandmother of a 4YO girl & 7MO boy, both from the marriage of my 29YO son. I've watched my granddaughter while the kids work since she was 2MO and loved every second of it. At the time she was the only child in the house. I'm now watching my grandson as well. Both the 4YO girls do go to school (when school is in, anyway - I did tell them before they had the baby I could not watch 3 - School holiday's were never mentioned ...Ugh). My daughter goes 1/2 days, granddaughter full days. My granddaughter arrives about 3PM and her mom picks the kids up around 5PM. I've recently been diagnosed with arthritis in my knees. I just can't watch my grandson any longer. It's so hard for me to carry him up and down the stairs. I've tried putting him to sleep downstairs but he won't go to sleep down here. Plus, my husband works from home and his crying disturbs him. He's a lovely boy but screams instead of cries and can be very hard work. I have to tell my kids I can no longer take care of them. I feel like a failure. How do I tell them? Thanks for any advice. Lori in Florida P.S. - they do pay me $300 a month to watch the kids.