How old is your daughter? First of all, fishkeeping is extremely addictive. You'll start out with 2 small clowns and end up a couple of years down the road with a 120 gal reef.
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It's also very time consuming to set up a SW tank. I've heard that the maintenance slows down as the tank ages, but I'm only about 2 months into it. SW is different than FW. With FW you set it up, decorate it, and that's pretty much it. With SW, you're adding live rock and your clean-up crew, testing the water, doing water changes, and forever seeing that one last fish/critter you'd like to have. Then you have to research what will get along with what, what special needs it might have (I just now found out that my scooter blenny needs pods to eat!), etc. And every improvement you make, your tank gets that much more beautiful!
It would be a good idea to put some serious thought into how long your daughter is going to want that clownfish, how much help she'll be in taking care of it, and how much time and money you're willing to invest in it. Do YOU want a clownfish (because we all know we end up taking care of the pets we get for our kids)?
As far as a hobby for either your daughter or yourself, I think it's an excellent one. I've raised everything from horses to gerbils to fish, and there are few things more rewarding than keeping another living thing both healthy and happy.