It's actually important for parents to have pieces of their lives that aren't 100% about their child(ren), because one: devoting your entire self to your child and taking zero time to yourself is a recipe for exhaustion and burnout, and two: kids grow and if you don't have a self to return to you end up feeling left behind (or you turn into one of those weirdos who tracks their kid's phone while they're at college to make sure they go to class and thinks it's appropriate to go to their job interviews). Parenting is the vocation where you're primarily working to put yourself out of a job, after all. Even if you've got a special needs kid who requires constant care, please set up a support network so you can go out to dinner with your spouse or play board games with your friends on a semi-regular basis? Being a martyr does no one any favors, least of all the kid.

Also, the thing where LW is like "for a long time it was her and my son's business and now she's asking ME for CHILD CARE" sounds to me like DIL is possibly, oh, idk, adding more activities as her kids get older? Or maybe Son is adding activities as well, and there are some spots where they're both busy?