Are you planning to pool multiple surveys? I would not recommend that. It will be best to calculate the prevalence of anemia separately for each survey, and not to calculate an overall prevalence for all surveys pooled. The combination of surveys does not correspond with a well-defined population and the dates of the surveys are different. But if you do want to pool, I recommend that you give equal weight to each survey. If, instead of that, you weight in proportion to the size of the country, your pooled estimate will be overwhelmed by the largest one or two countries.

If you want to weight in proportion to the country sizes (which I do not recommend) then you can use the population estimates for the number of children 0-4 (or other subpopulations) provided by the Population Division of the UN, for July 1 nearest to the mean or median date of the survey. The relevant number of cases in the survey would be the non-missing cases. That would also be the relevant number if you want to weight each survey equally.

The Stata commands for weighting in proportion to population size or for weighting each survey equally have already been posted on the forum, but let us know if you can't find them.