It wouldn't necessarily be about reducing contact time though, but not having everything revolve around the kids while they are there, which is what happens every fri-sun, from what the OP says.
We have DSS every second fri-mon, and he is not glued to DP's side within the whole of that time. He'd love it if DP (and/or I) spent every waking hour playing games with him and doing what he wants to do, and we do do a lot of that, but he knows he also has to entertain himself sometimes while we get on with boring jobs, or have discussions, or have friends over to dinner or whatever.
But it's true that making a change like this now would require sensitive handling from their dad, and if he hasn't been motivated so far to encourage them to do their own thing more, I doubt he is going to change now.
TBH if I was OP I would also want to have some flexibility of the odd weekend (or at least weekend day) without the kids, to visit her family/friends, or go away somewhere or do things that interest them as a couple but are boring to kids. Is this just totally off the table OP? Is it that DP doesn't ever want a single weekend day without the kids, or that their mum or GPs would refuse to ever have them, or both?