Okay, this is really tough. What do you define as long term? Nothing upset me more (before we were engaged) that some people didn't consider FI and I "long term" because we were not married or engaged or whatever, even though we were together for longer than a lot of people, and living together and everything.
The way we are doing it is that, if we know you and your partner is long-term, the invite is addressed to both. Otherwise, just you. NO ONE is being invited who will have no one else there - family or other friends or whatever - but that just luckily worked out with our friends and family.
Does that make sense? I think you do not need to add an "and guest" if you do not already know WHO exactly that guest is going to be.
I do agree with torilynnsmith - would YOU want to go without your partner? Or your kids? Also, I LOVE the idea of supplying childcare - I have seen this done and it works out wonderfully!
I do not want this to come off as rude; just remember not to stress - it will all work out!