You are correct that the data are for children born in the 3 years preceding the survey or the five years preceding the survey. This information is returned in the field ByVariableLabel and you can use that field to then filter the data. Alternatively you can use ByVariableId as follows:
ByVariableId, ByVariableLabel
14000, Three years preceding the survey
14001, Five years preceding the survey
To get just the data for children born in the five years preceding the survey you will need to access the data for both first and then filter on just the latter. There is no way at present to filter the data at the time of the call - you need to do the filtering after you receive the data. However we are looking at putting some more advanced filtering into the API in the future.