I guess try and get something easily changeable? Like - with her name, what about buying that in wooden letters (since that will never change, lol), and the colour can be repainted anytime. And then cheap canvasses covered in material - so you could browse the quilting section of a craft store for some lovely butterfly material, and if at 3 years old she decides she hates butterflies, no biggies, just replace the material. Or frames with prints in them, and you can just change the prints. Or cushions where you can change the covers.

There are probably lots of cheap decorating ideas, homemade bunting, things hung from the roof or along the wall with string or ribbon, mobiles...

Having said that, I'd say butterflies is fairly safe....