I guess the lambs won’t be eating a lot of grass for the first week or three, so maybe it wouldn’t be too big an issue? You’ve wormed the mother, so she won’t be shedding eggs, pregnant ones won’t be shedding much until after they lamb - whereupon you’ll worm them.
Lots of farms, including all three I’ve lived on, sometimes leave some lambs in the field with their mother and the group they’ve been with all along - maybe it’s the group of singles, maybe it’s the last six or nine ewes in a group that was much bigger at the outset, or whatever. And my whole fleece flock up north stayed pretty much together all year round, ranging over their 25 acres of mixed pasture and woodland.
So I wouldn’t get too stressed about it
. Sure, if you have the setup, it’s ideal to put the newly wormed mothers and their new lambs onto fresh pasture, that didn’t have lambs on it last year if possible. But if you haven’t, just put them where it makes sense. Sounds like you have other livestock, so you probably manage to do a bit of rotational grazing, where each species eats the worms of the last incumbents, so your pastures are probably pretty clean anyway.