Do you have a trial period - where you can terminate care if it's just not a good fit...
I'd maybe offer a teddy or soft music - something else to make up for the 'lost' bottle OR try like the ladies said above...try with water instead.
The BIGGEST Problem is the parents are insisting - I'm sorry but I make the rules here, if they don't like it then they can find someone else who will let them make all the rules. This is just the begining.....if they're already telling you how to run your home and "make" you change your original rules....good luck in 5/6/7months from now. What else are you going to change for them because they are "insisting"?