Kerry if you let the restaurant manager know a couple of days in advance that its your daughters birthday they will usually bake a cake or you could mention it when you book one of the speciality restaurants and ask if they can organise something, I have seen many birthday parties in DR, the staff often dim the lights to bring in a cake singing happy birthday in Spanish.
As for spending money, if you have to buy extra sun protection or mossie stuff it will cost a fortune so take plenty with you, also if your kids expect crisps, sweets or chocolate, again the prices are extortionate (£1 for a Snickers bar
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) so take a few snacks with you but beware that chocolate will go very soft unless you have mini bar fridge. You might also like to take some small bottles of water for when you land, its very hot and getting out of the airport can take a while but get the kids to carry them in their hands while they weigh the hand luggage or you could be overweight. If you save the empty bottles you can fill them in the restaurant for the return trip, airport prices again are a stinger.
One good tip is to pack something (for me its my pills) in a tupperware type container so you can take food back to your room and the ants won't get into it. Its handy if someone is poorly or too tired to go for food, don't make it too obvious you are taking food from the restaurant as some don't like it but if anyone says anything just explain that one of your party couldn't get to the meal. I have to do it all the time for my elderly Mum and its never been a problem as I don't take crockery. (you will have to sneak cutlery though
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)
Have a good time, the DR is great.