I've got a pump with tubes and I occasionally wear dresses.
I try to get dresses that have pockets (in general, not just because of diabetes) and I sew large button holes into the pockets that the pump can slip through. The tubing can go from my infusion set, under my clothes and into my pocket, making it easy to take out the pump when I want to check where my blood sugar is at, when get an alarm and when I need to bolus. Having the pump in a belt under a dress would mean needing to lift up the dress to get to the belt and the pump.
The dress modification might sound like a faff, but it was easy and makes life with a pump a bit simpler for me. Any seamstress should be able to do it if you explain what you want.
I do have a pump belt (Spibelt, they make kids sizes too) but I use it at night so my pump doesn't get tangled when I sleep. I do use it around the house during the day sometimes if I'm not wearing something with pockets, but more often I just tuck my pump into my trousers, shorts or skirt.