Nice outfit choices. First and foremost do what you are going to enjoy, hopefully something they'll enjoy and within their limits so they want to come back for more.
Top one has a really high neckline but that shorts style bottom should keep in lots of mess. If it were up to me, I'd probably generously fill the back with custard until there is a nice bulge going on then sit them on a pie or a cake and watch the look on their face as that custard works its way around the front. Once sat you could then cover them top to toe and possibly fill more through the arm holes, because getting in at the front is going to be tricky.
Bottom one I'd have them wear slightly unzipped to funnel in any mess. Given its sleeved but has no legs I'd be half tempted to try something a bit different and have them hold their arms up (tied if you're into that, I know not everyone is) and fill the sleeves until it trickles all the way down (probably custard as a go to). Suspect it would take a while and be quite tickish for them. Then tip a big bucket over their head, most making its way into the outfit. Having them slowly zip themselves back out of it and watch the custard begin to pour out at different rates at different levels would be fun.
All just ideas, ultimately do what you enjoy and I hope you both have a great time.
Love you, too
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