Take the same precautions you would take at home. The bars are all friendly, but of course two girls 17 and 18 are targets for guys wherever they go in the bar scene. Some of the more cool scenes are Barefoot Iguana, Fidos, and Big Daddy's for dancing and music. Other good beach bars are BC's Beach Bar, Coconuts Beach Bar, The Purple Parrot at Ramon's Village, Pier Lounge at the Spindrift Hotel, etc.
The only suggestion I would venture to make is to make sure they travel to and from town by taxi or some other pre-arranged method. This is safe and a good way to travel in AC late at night.
If the girls were to stay until closing, they might end up walking home at 1, 2 or even later in the morning.
Having said all that, I am sure you will have no major problems. Belizeans are friendly and helpful. Crime is almost non-existent compared to home in the States or Canada and usually only consists of petty theft.
Make arrangements with a taxi driver to take them and pick them up. It might cost a little more to keep the taxi driver awake but it is worth every cent.
This is my take on the situation re: 17 and 18 year old girls but I'm sure that Sandshaker, Jesse, Lan, Chloe, or Choochoo might have some input here.
Enjoy your vacation and just use common sense where the girls are concerned.