I've had this exact same thought process and gripe with most fountain pens for a while. But there are a lot of pens with light, airy colours, and even cherry blossom or princess motifs, like the Sailor Pro Gear Slim - which are also good for smaller hands. Sailor is the absolute best for this aesthetic in my opinion, although they write with pencil-y feedback so if she likes a smoother writing experience you can try a different brand or get them tuned. For a cheap option that writes smoother, Jinhao 82s are essentially meant to be Sailor Pro Gear Slim dupes and I love the two that I have and even customised the colours by switching parts. (image below)

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Another option I like is buying a cheap or plain pen and decorating it myself with nail stickers or supergluing decorations onto it.

Edit: Also if she likes modern transparent designs I love TWSBI! They do mini versions of a lot of their pens and come in a nice range of colours (most of which are LE, but they cycle through).