So I realize that this may be taken as me stepping into a parental area that I do not belong, but I honestly don't mean it that way. I'm not trying to butt in, promise! I'm just thinking that maybe there's a way to work this out so everyone wins, fish included. So, some thoughts... maybe you could get the bigger/taller tank, and try alternate routes with your girl. My little girl uses a step ladder, and I have her put her hands on the top of the ladder as she climbs on. You could even mark the hand spots in her favorite color paint? Or with stickers? Or you could have her climb on a desk chair and, while holding her securely, roll her to the tank. (Of course, you'd have to keep holding her as long as she's on the chair.) Or could you fashion a place where she could safely put her hands that isn't on the tank? Like a broomstick or something that's been placed at about the height of the tank rim but is secured to the walls or tank stand? Or just lowering the stand, either by modifying the legs of it, or using a different stand? Since she loves the fish so much (which is adorable), maybe if she understands how her actions could hurt the fish, she'd try hard to be careful of her hands. I don't know, just some thoughts.
My "Aquarium Assistant" is 7 years old and, like you, I'm so glad we have this hobby together! Good luck!