Op, I hope you meant that you are driving, not flying. You weren't thinking they would allow an inflated Mickey balloon in the aircraft, did you?
Southwest actually did allow DD's mickey balloon on the plane. Now, before I say more, we honestly just hadn't thought about it before we got there. If we had, we would have given it away in the park or deflated it. No one said anything as we went through security either, or again we would have done something with it then.
We didn't really think about it til we were boarding and were like, crap, what do we do with it now?
anyway...SW did let us board with it. No one, passenger or SW employee, actually said anything about it to us. We had bought DD her own seat so she could rear face on the plane (she was 15 months old and still under height and under 20 lbs at the time, so not allowed to FF). We ended up putting the balloon in the space under her car seat, where people's feet usually go when you're sitting FF. The car seat kept it in place, and I guess it had deflated enough that the pressure change didn't effect it. DH and I were the other 2 people in the row with her, so it wasn't affecting anyone else.
To be clear - I'm not suggesting this as something that should be intentionally tried...as I said, had we stopped and thought about it, we would have done it differently, it just didn't cross our mind til we were on the plane. We've never gotten those balloons before, so we didn't think about it. It worked out, but I definitely wouldn't make a habit of it.