Best castle views are probably going to be from SagoCay and Conch Key. If you booked lagoon view and had it noted to see the castle from one of those buildings, you might get your request. It's not guaranteed though, even with a lagoon view. You could take a chance and keep your garden view booked, but put in a request for a corner room. The corner rooms are a little bit bigger and come with two balconies. That usually means a garden view and a lagoon view. Otherwise, just leave your reservation as is and ask about the view when you check-in. If they have a castle view available, they can either just give it to you, offer the upgraded view for a discount or quote you the difference between the rack rate of garden and lagoon view.
To be honest, all the views at the resort are beautiful and all the buildings offer something unique, either privacy, or closeness to the pools, etc. I would suggest that you just leave the reservation alone. If you have the extra money in your budget, I'd book some of the grand adventures offered instead. I think they run about $30 per child and your DD can have tea with Alice, or make some tasty treats with a GF chef. Or how about booking a mom & me manicure at the GF spa?
We've had both castle view rooms and non-castle. We definitely didn't feel like we lost any of the magic by not seeing it from our room. My DD8 thinks of the GF as the princess hotel anyway. I don't think you'll be disappointed with any of the rooms. If by chance, you're not happy with what you're assigned, just head back to the front desk and see what other options are available.
Tammi