Ok, i only skimmed the responses, but i think my response it not going to be very popular. But, here goes anyhow.
My mom and my mil are very different people. My mother was a single mother of 4, her parents died young, so she had NO help. her ex husband was a schmuck...she had no child support, no money, worked two jobs, etc. so, seriously, no help. My MIL was married to a marine, so not much help from him because he was usually stationed away from her. anyhow, her parents always helped..my husband is an only child, so it wasn't hard to help. With that being said, my mom works fulltime and always has...she offered to try and figure out a way to retire early to be able to watch my three kids....she understands what it's like to have no one to help. plus, i don't know about NY, but here is CA summer programs are extremely expensive...back to my point. my MIL is still married, does not work, lives less than .25 mile from me...(my mom lives 30 miles from me..and in CA traffic we're talking an hour), and for example has been visiting relatives for a month and a half, got home on Monday and has yet to see my children. have i mentioned she lives 5 minutes from me...walking. every summer I struggle to find childcare for my kids. three kids in a summer program is $4220 for the summer. i don't have that. so i lean on friends and family. my mom takes vacation time to be with my kids for a week or so, and my MIL can't even take the time after not seeing them for more than a month to come get a hug even. my three year old doesn't even know her very well.
so, fair is not the right word...he isn't asking you to watch them all day 5xs a week for the whole summer. he's asking you for 8 hours a week out what maybe 60-70 awake hours a week. is it really that much? i don't think you should have to rearrange your work schedule for him to work out, but if you have the opportunity to build a loving, trusting relationship with your grandsons, why not? as a parent who works out 6xs a week, (after the kids are in bed, thank you to my husband), the work out is a release for me. i live a very hectic life, being a driver, counselor (i have three girls two on which are tweens, nuff said), cook, clean, manage a home, my husband and i both work FT out of the home..i need that hour and half or ill go crazy. i have dedicated my life to my children, but if you don't leave a little fraction of the day for some r&r regardless of how you get it, you'll lose your mind.
don't get me wrong, i don't get upset with my MIL for her lack of interest in my kids, i know she does the best she can with what she has, and raised an amazing son, that I fall more and more in love with everyday. i appreciate her for that. but really, without my mom's help, past, present and future, my life would be very very different. she comes over every friday to hang with the kids so my husband and i can go grocery shopping, run errands, etc etc. those few hours makes my life so much easier...
my point is, fair is not the right word...opportunity i think is better. you have an opportunity. does it have to be etched in stone four days two hours a day..no. take it day by day...have your son call and check in, hey got time for my work out day? generally, yes...but sometimes you'll say no. maybe he can workout around your schedule?
also, the daycare at my gym sucks...just a bunch of teenage girls standing around chatting. i don't take my kids there.
again, i can't stress enough how much my workouts mean to me, sounds stupid, but it's true. i also can't put into words the appreciation i feel for my mom..she is my hero..
i hope this gives a little different perspective...and honestly, if you choose not to do this, thats ok to. people build relationships in many different ways. :)