I don’t think the people on here mocking the uninvited friends for being put out or calling them/the OP immature are especially helpful or necessary.
In my experience there are two types of kids parties.
a) very much for the kids and their friends - tends to be when the children are a little older and have active friendships under their own steam. Often activities/at venues that are very obviously ‘kids party’ eg soft play, carting etc. The party is the activity so to speak and the adults are there as chaperones not guests.
b) parties which are equally for the adults/the parents wider social network. Often when the child is too young to have any active friendships and the child guests are therefore the offspring of family and friends. Tend to be less obvious ‘kids party’ eg in a church hall, home etc. Whilst there is obviously a focus/activities for the children you’ve also got adults catching up over a glass of wine/coffee etc rather than being there purely in a capacity to stop child a ramming child b with a go cart. The adults are there as chaperones and guests.
The venue you’ve picked puts it more in the type a category for me but if you’ve always had type b parties then I can see why friends might be upset if it’s not been explained that this year is different. Once friends start having children it is understandably harder to get the group together/see everyone - how many posts do we see on here of women who feel their friends have ditched them since having kids (or since being the one who’s had a baby). It’s a difficult stage to negotiate in life (especially once you throw in people having fertility issues etc) and not to sound twee a bit of kindness and not assuming the worst of intentions goes both ways. I can see why they’re upset if they think you’re having party type b but have only invited people with children, especially if they if they have made an effort to be involved in your and your children’s life. Especially if they hear about it second hand (eg two of your friends with children mentioning that they’ll see each other on Saturday at DC’s party).
You’re completely entitled to have the party you want and given your circumstances I think you’ve made a very sensible choice but I think something has gone amiss in communication which is why your friends without children are upset.
I’d message them something along the lines of ‘sorry if you’re a bit upset re DD’s party. With it being so close to my due date we’ve decided to do something small and simple this year rather than our usual party (just in case we need to cancel!) so are going to go to soft play instead. Unfortunately this means that grandparents, family and friends aren’t able to come. I really value your friendship and support and hope you understand - would be lovely to catch up soon/see you at so and sos/blah blah’.
If they’re still upset after an explanation then they’re tipping into unreasonable territory but I think it’s worth making sure everyone’s on the same page. Hope the party goes well and best of luck with the rest of the pregnancy!