I think Lori's response is absolutely brilliant and a great way to turn this into a family event and a demonstration of giving!
I sympathize with your dilemma - but your parents' relationship is not going to change after all these years. If your mother is a negative "doormat" type, she's getting an emotional payoff from the martyrdom. On the plus side, she's not sitting in the nursing home all day, and you father either has to be nice to the staff or he can at least be domineering to someone other than your mother. If she feels guilty for not being with him, or with you all, that's her problem.
My mother spent a lot of time in a rehab nursing home after a surgery, and she was incredibly difficult. But there are tons of people there who have no one and I think you could do a lot with the kids to "decorate" Grandpa's room (and, by extension, the community or activity room) for the benefit of other patients/residents. Kids of all ages can make construction paper tulips & daisies - you can take a bunch there, or you can talk to the staff ahead of time about having the kids make some with the other residents at a bunch of the smaller tables in the activity rooms. Take paper supplies, scissors, markers tape, glitter, glue-on shapes or stickers, etc.
If any of the kids like to sing, rehearse a few songs and have them go "Spring caroling" from room to room - they don't have to be Easter songs or even spring songs - just old American favorites that all will know ("Take me out to the ball game" or "Do-Re-Mi" from the "Sound of Music", anything they know).
Talk to the staff ahead of time about whether you can bring homemade cookies or (for safety reasons) store-bought goodies. Not everyone can eat them all due to diabetes or other health issues but maybe you can get some guidance.