I'm no help with the syrup but I do have a sneaky cheater way to do it. You might have to phone your mum in advance so she can organize it though.
Instead of shaving regular ice cubes then adding syrup, make up cordial and pour that into your ice cube trays. We used to do that as kids cos we found it tasted better and you don't get a big melted patch where you poured the syrup on. Another cheater way is to buy zooper doopers and shave them, this way is a tad more expensive then cordial ice cubes though.