My son and DIL would love me to have their children overnight. To be honest I struggle to look after them during the day for short periods. One is still in nappies. I think there is a lot of pressure on us grans to do childcare, yet we've been there done that, got the t-shirt.
As you get older it does get more difficult, for whatever reason. It's just how it is. When you're young you cope. You have to, they are your children. Grandparents are not obliged to look after small children. It's so much more difficult for us, yet no one seems prepared to acknowledge that fact!
I always refuse to have any grandchildren to stay. I just couldn't cope and I refuse to feel guilty about it. End of!!!