I think that you are being unreasonable, OP with the slightly ironic username 😉 because you did invite them. Do you live in the UK @ImStayingInside because I think it is rather more unreasonable to take all of you, but particularly an 8 week old baby, to a park for a picnic when it is this cold?! A brisk walk, and then pop into the cafe in the park for some soup, or something, and to be able to feed baby and change its nappy in the warm, would be much more my idea of a nice little outing with the newly increased family. But each to their own, your DH and DD1 could presumably have spoken up if they didn't fancy the picnic idea (I think that you are maybe a very outdoorsy family, which I think is great) and if you are breastfeeding baby, then it's your breasts that will get cold. I am not worrying about baby getting cold, as you sound way too practical to have overlooked anything to do with your baby, so I suppose I think that you are mainly being unreasonable to yourself!
IMO the joint most unreasonable ones here are your DB and your DM. Firstly at 25 (not 5, you didn't accidentally put a 2 in front of his age did you?) your DB is far to old to go complaining to his mummy that you have asked him to bring a few things for him and his girlfriend to eat at the picnic, but even worse than that, is that he said he would feel stupid bringing food to a family picnic - that just does not compute, why on earth would he feel stupid, I can't imagine anything more normal than bringing some of your own food to a picnic?
As for your DM ringing you up to tell you off (especially as you are only 8 weeks since giving birth), it would be laughable if it wasn't so infuriating. For her to then suggest say that you could just make your DB's DG, something for her to eat, is turning this small, rather sweet (even if it is also a slightly mad imo) plan, into a bloody military exercise 😂
Did you forget to tell your DM that you had a baby 8 weeks ago OP? Did your DM, DB and DB's DG not think that as your DB and his DG were going to be meeting his DN for the first time, and as she is still very young, that when he asked if they could visit you tomorrow, they should have been offering to bring a stew or something for you all to share, or a takeaway if that was too much trouble for them...
@ImStayingInside May I please forbid you from going out again to get more food, but this time for DB and DB's DG? The upset and miniscule confusion - all things considered - caused by you not making clear at the time, that on this occasion they would be expected to bring their own food to the picnic, does not warrant an 8 week post-partum mum having to go out to get her 25 year old DB, and his 25 year old DG some food for a ruddy picnic. They know the score now, so your stupid DB, and his probably just as stupid DG either bring their own food to the picnic, or they go hungry, or they decide not to go to the picnic at all, and go instead to your DM's for lunch, as I am sure that DM is eager to make amends for her disgraceful DD, who apparently can't have understood the picnic etiquette rules that DM has drummed into her over the years.
I am also sure that your DM will love to go to the extra effort to make sure that her DS's picky DG gets something to eat that she will enjoy. I just hope that DG remembers the etiquette rules that her own DM hopefully drummed into her over the years, and that she therefore remembers to thank her DBoyfriend's DM for all the effort she made making her DS's DG's picky meal.
Enjoy yourself tomorrow ImStayingInside, and while I'm staying inside I will be thinking of you, please have some 💐