I know this isn't going to help the OP (mind you, neither are dozens of posts agreeing that £25 per child is unheard of and that she should cancel ...
) but I've never really got the whole big splashy costly party. They have a blast just hanging out together and playing some games. You also do NOT have to invite the whole class, and many kids prefer to invite just a few friends, which obviously keeps costs down. A few suggestions for low-cost options:
-If you have a garden, then use it! Scavenger hunt (leaves, feathers, orange pebble, straightest twig etc), treasure hunt, making dens or camps.
-Go to the local park/playground. Enlist in a few parents, let the kids do park things! Ask the park authorities if you can bring a gazebo and if they allow serving food. Cool boxes with ice lollies will go down well.
-Go to the woods and make dens from branches etc.
-For older kids, rent or borrow 'laser tag' guns and let them loose. Or water pistols.
-if they like making things, get some cheap craft supplies and have them do something like decorating a bag that they get to take home, making necklaces or key rings, even those 'paint your own pottery' party kits you get from the paint-your-own shops are cheaper than soft play.
-Take a small group to a fun local museum if there's one, treat them to lunch in the cafe (normally will have kids meal options. It's still going to be ££s cheaper than the overpriced shite they serve up at soft play). Most museums now have great stuff on offer for kids and this is not a 'boring' option.
I'm perhaps lucky as my son never wanted a whole-class party (we did this just the once, hiring the village hall and a local and well-loved children's music entertainer. It was ... an experience, and this was a pretty laid-back event. DS didn't feel at all well on the day so remembers NOTHING!
). We've always just asked a handful of friends. When he was younger we'd do a craft-based thing along the lines of shared interest (Star Wars craft mostly - make a lightsaber or utility belt parties - not at all hard and loads online to help). A bit older and we'd go to local woods, fry up some sausages and they'd play capture the flag type games. Or go to the local laser tag and not book the party option, just book into the 'play for all' session, do a couple of games, and bring them back to the house for pizza and maybe a DVD.
Kids' parties don't have to be mega-expensive and I think if people dialled it down a bit, then expectations of a massive Event would drop to realistic proportions.