Your situation changed and they haven't recognised it. Going back to work is the perfect opportunity to reset boundaries without hurting feelings.
I would start saying things like, 'it's been so lovely being on mat leave and having the flexibility to see everybody so often. I'm going to miss it when I go back to work'. Lay the groundwork now.
Just before you go back to work, get the calendar out and block off whatever time you want to keep free. Show the GPs and say confidently, 'let's make sure that we still get to see you now that DH and I have so much less time. It won't be the same as before, but let's do our best'. Get some dates put in when you can see each other.
When challenged, be clear that lack of downtime will be detrimental to everyone. And that the time blocked off isn't up for explanation or discussion, 'that won't be possible, I'm afraid'.
If you feel comfortable letting the GPs look after your DC, get some dates put in when they can take the DC out to give you a break.
If they kick up a stink, be clear and firm that the situation has changed. Express surprise that they thought things could continue as before.
Not easy, but setting firm boundaries now will save years of stress.