I have never seen lactose free milk out and available on the buffet or in the dining room, but that does not mean that they do not have any onboard. I would imagine that they just keep in somewhere separate and that you would just have to ask for it.

 Your best option is to  either fill out Royal's special needs form (it does have an option to select Lactose-free milk) or send any email to the special needs department  special_needs@rccl.com, with your name, ship name, sail date and reservation number (at least 45 days in advance if possible).  When you contact special needs it helps them know in advance when anyone with any sort of allergy or special need is on board that way they can be more prepared when you board the ship to make sure they have  something to accommodate your daughter.