For some reason I thought of your post last night and realized that the "guys" in our two parties for our next trip are about the same age. My brother is 36 and my DH is 50. (well, I guess he's a little younger than your Dad

) Anyway, I really never thought that they would really even want to do anything other than what we're all doing.
Did "they" express the opinion that they'd be bored??? You do already have a "boy/girl" day where you'll each being doing something special for the "sexes".
I'd just plan a little somthing in the day for each of you. Like make lunch the ESPN Club one day. That's what I usually do. Personally, I might be wrong, but you sound like a very nice family, and I'll bet the most fun you're all going to have is watching the antics and reactions of those two little ones. I KNOW that's what "I'M" most excited about for my next trip, is doing this with my neice and nephew!!!
As for the lines. Take suggestions from the wonderful people here on the boards. It's invaluable. Don't stand in the long lines. You certainly don't want the two little ones to only remember long lines and heat.
That's exactly what happens. I've been there and done that, and if mine were that little again, I'd just enjoy the little things with them and not worry about big rides etc. Characters, jumping fountains to run through, just the magical atmosphere, get in some rides early before lines and then use FP and plan to get in only what you have time for without lines.
Make your priority list including something for everyone, but mostly stuff for the kids. It will make not only the men happy, but your two little ones too. And you already said the ladies will be happy no matter what you do. Personally, I think the ladies would be most happy without whiny kids and men!!! LOL
We also went when we were in our 30's and in-laws in early 60's and we all did everything together and had a blast!!! There was one exception, on the last day, we had reservations at Cindy's and my FIL and MIL decided to eat a second time at the Diamond Horseshoe in MK, while we ate at the castle. They loved the show the first time we saw it and wanted to do it again, especially my FIL.
They really might surprise you. One mistake I tend to make ALLOT when we take other people with us, is worrying and fretting so much about THEIR time at Disney World, that I forget to have a great time myself. I'm already doing that for July and you've reminded me that I gotta calm down and have fun too. Thanks, there's allot of good suggestions here of things you could fit into your day for them, ask them if they'd like to do any of them. I'll bet you all would enjoy them too.