Hi Jenn & Welcome!
What a sweet little girl you have. I'm sorry to hear she has reflux and motility issues. I'm glad to hear though that the Reglan is working for her and she's not having side effects. No dairy allergies/intolerances is great also!
As far as Prevacid versus Prilosec, it just depends on the child. Some children do a lot better on one drug over the other. Prevacid is compatible with Reglan as long as they aren't given at the same time (at least an hour apart I believe). Also, Prevacid can act as a mild motility drug in some children and will reduce refluxing. I don't know what percentage of children that happens in, but I know with my youngest Eva it greatly reduced the amount she refluxed. I would go ahead and give it a try.
It's important that it is given at least 30 minutes before a meal on an empty stomach to work most effectively and it can take up to 14 days to kick in and really start working.
Let me know what you decide and how she's doing!