Tbh, I can see both sides of the problem.
Usually, I agree that older siblings should not be taken to the party, and if my youngest is invited, I would normally arrange to leave my eldest with family / a friend etc...
But, As the OP has said..."I'm not expecting any parents to leave the children - they're only 3 or 4 and it's quite a drive."
This makes it quite difficult for the other mum to arrange alternative childcare. Its a farm, so not safe for the parent to leave the child at the party alone (as you could probably get away with at a hall)
"Its quite a drive" - so I'm guessing quite an ask for the older child to be left with someone else too. Its not the case that the mum could let the child turn up a bit late to the party, and squish it down so a friend has only got to look after the older child for an hour, but if it is a standard 1 1/2 hour party, with say a 20 min or so drive either side - that's finding childcare for almost 2 1/2 hours. That's quite a big ask!
Therefore, I think If I were the OP, I would probably recognise that my choice of party is actually quite inconvenient for other mums, and would probably allow the other mum to pay for the older sibling to attend this time. If it were at a local hall, I wouldn't see the need. But given the nature and location of this party, I think it can be inconvenient for other parents to drive to a farm, some distance away, and perhaps there does need to be a bit of compromise there!