R.R.
You'd have to pay but pharmacies will add a flavoring to the medicine, vanilla, chocolate or bubblegum would go good with the mint. But since you already have the Zantac...
1) Buy some Yo Gabba Gabba! Medicine-time Flavor Spray, made to coat a child’s tongue and mouth with special sugar-free, dye-free sweetening enhancers in Dazzleberry flavor. By covering the tastebuds of the tongue, it helps mask the bitter taste of medications. You spray the product in her mouth before giving the Zantac and then again after. (I'VE tried it, it works!) And it's made by the people who make the flavoring that pharmacies use. You can order it on Amazon:
http://www.amazon.com/Yo-Gabba-Medicine-Time-Flavor-Spray...
2) Give your daughter a popsicle before giving the Zantac, they make mini ones now that are the slow-melt kind and the perfect size for this to help numb her taste buds and make it easier to take her medicine.
OR
3) Give her a piece of strong peppermint gum to chew before giving the Zantac. This will help disguise the taste. If you don't want her to have gum, say before bed, have her hold a piece of mint gum or a strong mint (that maybe you'll let her eat afterwards) under her nose as you give the Zantac. I know, it sounds crazy, but as we know the smell of medicine (antibiotics come to mind) can actually make it taste worse so this may help.
I have reflux and hope this helps and that she feels better soon!