Hold on, is your friend doing you a favour by coming to help set up? Is your extravaganza that you get all the glory for, but which she helps you with all the setting out of things, or is it a shared effort for which you both get recognition that just happens to be at your place, because that would affect my view of things. In the latter case, of course she should do her part, but in the former, you are asking her to put herself at an inconvience just to suit you.
It sounds to me like you haven't taken your friend's changing circumstances into account and are expecting her to turn up for what you view as a 'fun time' as if she didn't have the responsibility of a two-year-old and, to a lesser extent, a husband to worry about. And I'm afraid I strongly disagree with your twist on 'since they have a child, they don't get to do everything they want.' You seem to think it means YOU can have what YOU want instead and they should be used to making sacrifices anyway, so one that happens to give you your way is going to be fine.
Sorry, but no, it doesn't work like that.
Besides, since you have a DH, who will presumably be co-hosting the party and getting at least half of the credit, what's wrong with making him do the work instead?