My experience with an 8 month old.......
I brought my daughter to Sosua from UK in Sept 05 when she 8 months old along with my other daughter who was 3 and a half . The younger one didn't have injections as she was under age for them. She was just between crawling and walking and she was into everything. It's impossible to stop children that age from exploring everything without strapping them into a chair. Everything went into her mouth and she was constantly crawling all round the floor. We ate out most of the time, including Dominican food (they both eat anything and everything) but she was fine and wasn't ill at all. Without wishing to be 'blase' about it, I believe that a little 'dirt' etc helps to build up a child's immunity so I'm not somebody who believes in maintaining a 'sterile' environment for children and gets the bleach spray out the minute they touch something. We've been living here 8 months now without any illness or problems at all. However, my children are both very healthy and resilient and have never really suffered from any health concerns. Also, they're used to spicy and unusual food as I've never given them anything different to what we have. Also make sure you stick to bottled water for drinking.
I did bring childrens medicines for diarrhoea, plus fever reducing medicines (Calpol and Nurofen) but fortunately we never needed them. Although, they're both quite dark skinned, though with blonde hair, I also put loads of factor 30 to 50 on the girls and covered up the youngest with a T-shirt and sunhat when on pool and beach. A parasol on the buggy is a good idea for walking out and about as well. Also, if you can get a (safety approved) swimseat which floats in the pool, my youngest girl floated round in the pool for hours quite happily while I kept an eye on her, and it gave me a bit of a break while I was teaching the youngest to swim.....
I don't believe in giving young children medicine for maleria unless absolutely unavoidable. I used the insect repellant sprays which are suitable for youngsters and covered them up in the evening, burned mosquito coils etc.
I assume you're staying in a tourist area and I'm sure with sensible precautions you'll be fine. It's very difficult with toddlers and young children and I never had much of a 'holiday' over those two weeks...... good luck!
Sorry, I've just read your post again, and realised that you have
FOUR other children, so I'm sure you know most of this anyway.... Anyway, maybe the toddlers abroad tips may be of use..... Happy hols.........