If they feel that it has to be done in person, then throw out 3 or 4 days/times over the next week or so, and tell them that you'll host them at your house and make them a nice dinner (or, maybe splurge and take them out to dinner). Point out the obvious: you're already on a super tight budget, so a babysitter for 3 hours is prohibitive; that bringing the kids would make it very very hard on the kids and throw them off schedule for days, resulting grumpy/crabby kids and family; that if they want to run to X store, the stores that you're likely to need to run to are already on your side of town.
Then, if they won't budge, I think you need to either just suck it up and make it work. ... Or, just thank them and respectfully decline their help.
If this is typical, where they want you to accommodate them (and/or are being manipulative or controlling), you might consider this second option very seriously, since by doing this yourself instead, you setting very clear boundaries. Yes that means that you'll likely end up just getting married by the minister with just your witnesses present, and that's it (maybe having a BBQ potluck later to celebrate, instead of a "traditional" reception).
Personally, I think that the BBQ potluck sounds like a wonderful, fabulous time and more fun than a traditional reception, but that is just my opinion. :)