hello there
i do have an idea but it depends upon how good your school is and whether your kids are in the same school i.e preschool/y2?
when we emigrated - same scenario DH had gone on ahead - i got permission from the headteacher to bring in a magician for the afternoon.
The class teacher/TA were happy/had an afternoon in classroom to do stuff i.e. were present but could mark.
Chose a magician i had seen at local leisure centre - he was great - cost 75 quid and made balloon animals for the class/let them stroke the bunny rabbit after the show.
Class all brought in a party item from home i.e. crisps cake biccies and we all ate these naughty things after the show.
Kids took home the balloon animal and i gave them all a magic note pad with new address on ...very cheap from a poundstretcher type place.
All in cost about 90 quid only cos i bought the magician a crate of beer on top.
If you have two classes you could amalgamate them/use a large classroom after lunch (go in and move the tables to side) or use the hall (pending drama P.E etc).
Teachers tell parents about the free magic show - as you foot the bill for that - but can they bring in a packet of jammy dodgers...
Hey presto!
BUT i was lucky - i knew the school staff pretty well and had already got them on side by giving them loads of old toys for the PTA sale before we moved. They could have said No though, it's mid term, too chaotic etc but i figured no harm in asking.
If this isn't a remote possibility then i would cut out your party bags or certainly the budget for them. One mum i know gave each child a new pencil taped on a bookmark with new address on....bought in bulk packs, a lot cheaper than small items per child per bag (even at asda with a balloon, cheap yoyo, pkt of mini smarties etc it all adds up)
Last idea would be to see whether a wacky warehouse type place would cut you a better deal than usual for bringing 50 kids. This is a tough one as i know there are usually limits for a normal party and with food and balloons at 8 quid a head that is 400 only saving you 50 but...maybe another similar place could do it for less or you hire the place out when they are not busy or on a slow time?
Personally would never take 50 kids on a trip anywhere without a whole amount of back up and sadly no weather to meet for picnic in park but the ballparks usually have soft area for little ones, climbing for big and staff on.
Good luck!!!