Where are you staying? Depending on the venue you might want to bring in Chefs For Hire and have a bash on site ? OR ...... if you want to do something totally unusual, do a daytime celebration by hiring a boat or two and doing a bar-b-q at the sand bar in front of St. George's Caye. That would be a real winner.
Dinner for 20 in a normal restaurant setting can be a little restrictive if you behave, and annoying to other diners if you don't ....... to really be able to mingle and enjoy each other I'd suggest taking over a space that is part of a restaurant, but separate enough that you can take your time and have a real birthday party. To that end here are a couple of suggestions ......
Elvi's has an annex to the dining room that you could take over for your group. You don't feel isolated from the rest of the dining room, but you do have your own space and can mix and play without feeling like you are bothering other diners. Great Belizean vibe and wonderful food.
You may also want to ask if Blue Water Grill would serve dinner to your group in the roof-top deck area above the restaurant's bar. If you can take this area over for your group you could have an amazing party.
For a totally roots-Belize party - see if you can take over El Foggon entirely. Tons of local atmosphere, great food, and just about the right space. Thatch roof, food cooked over an open hearth, sand floors ...... you can probably set the menu from their yummy specialties.