I might say something like this:
"In case you were wondering what to get my little one for her birthday, since they have all the toys they could ever desire right now, we were thinking of taking them all to Disneyland and wondered if you'd like to pitch in to pay for that?"
Say you'd like to do this but can't really afford it so you wondered if they'd like to help with that instead of giving gifts this time.
Include it on the list if they ask what she wants for her birthday, but that might be too subtle
"Would you like to help us pay for Disneyland for the little one?"
You don't want to be too demanding or forward, but you don't want to make the hint too easily overlooked either if that is what you'd really like to do. Just let them know it's something you'd really like to try to arrange. Maybe make it a surprise, like a surprise party. That way you can make it about the surprise more than about the "gift" but make sure they know they are contributing to this "surprise". ;)
Wow, am I a manipulator or what? hehehe
Once we took our kids to Disneyland by surprise. I have two, a boy and a girl. They are very close in age (14 months). We told them we were taking them to grandma's house. My daughter was ok with is, not happy, but ok, but my son pouted all the way there. Even though it took us 2 hours to get there instead of an hour like it does to grandma's. He didn't catch on for the longest time. Then as we drove up to Disneyland gates they got all hyper and excited. They were confused though. Um, weren't we going to grandma's house? Oh boy they never let us forget it. I think they had a great time.
Then we did it again for Magic Mountain. As we drove up to hill that leads to the gates they were like, um, this is grandma's house? We said, "Yeah, this is the back way". This time it was more believable because it takes us about the same time to get to Magic Mountain as to Grandma's. Grandma lives near there. It was so funny. Again my son fell for it, until he saw rollercoasters. My daughter may have fallen for it but she kept quiet again and just watched and waited. Then once she saw where we were she was like, HEY!
Funny their reactions! I loved it! They are too old to fool anymore though. Going on 15 and 16 this year. ;) Have fun with them while you can! :D xo