I use a Trist & Shout bag which I have on a Crop in Style trolley, which holds a lot of balloons, virtually everything except 260Qs, which are in the Trist & Shout bag.
I always take a folding chair with me.
I also usually have Elastoplast in case kids get injured.
I have spare batteries for cameras, in case the mother has forgotten hers.
A TV remote, which I point at kids - it's not remotely funny.
A latex hammer, to hit kids with.
Various other items - see my BalloonHQ column Box Of Fun from 2013
https://www.balloonhq.com/column/ilowe/jul13/and my Emergency Kit column, also 2013
https://www.balloonhq.com/column/ilowe/jun13/I usually take a hat, too. I hate to say it, but that was another column
https://www.balloonhq.com/column/ilowe/sep13/