A.N.
Good Morning.... I just read your post & felt compelled to write.
I would like to encourage you to go & spend time with your husband - all marriages need time away to recharge. You'll have a great time if you let yourself.... We have never had the joy of living near family so we've never had grandma's to watch our kids on a regular basis. It sounds like your mother in law is very generous in offering to keep your little one. I think maybe you need to cut her some slack.... your "first time mom" tendencies are in high gear.
I have three little ones (all under 5) & I can tell you that if we can go 6 hours w/o a diaper change- we're thrilled. Diapers today are super absorbent & keep the wetness away from their bottoms... there really is no need to change them unless they are soiled or super wet...... As far as not drinking as much - maybe he's having such a great time he's not all that interested. Toddlers eat if they are hungry & they drink when they are thirsty - he will not dehydrate himself.... especially if she is offering it to him on a regular basis.
You might want to try & relax just a bit & enjoy the time he gets to spend with his grandma. It's such a magical thing when kids get to spend quality time with them.... don't ruin it by having such hard core rules on things that are not "safety related". When it comes to their safety - I"m all about rules - but your concerns are more of a control thing...
No one will do things exactly like you do - but that's okay. She raised your husband (and maybe his siblings too?) so she's not a rookie. It's good for kids to see different things & to have different influences in their lives.... he will more than likely have a blast at her house & that's great!
I hope this all works out.