Papermoth's shop is full of images that can inspire a party or an entire room for that matter. For example, I could see easily see this print framed in a child's room inspiring a calming palette of colors from paint to bedding to furniture.
Since we're here for party ideas though, I would use this image as a jumping off point for a wishing theme. I am envisioning a tea party. For the invites, I would go with this dandelion stamp from Paper Source.
Imagine clear glass jars with dandelions inside and pale blue and yellow squares of paper around for guests to write down a wish for the birthday girl (of any age!). I would use all white plates and linens and have a tea party with pale blue candles and small bouquets of yellow flowers.
Like dahlias...
photo credit: Buttersweet