It is possible that for your eldest is refused residency as in the the eyes of Jawazat he is an adult.
If you manage to get him residency status then that would only be for as long as he is in full time education and up to max 25 years old. EAP for higher education is the cash equivalent of 2 full price economy return tickets per year, nothing else.
18-25 daughters can have their Iqama renewed up to the 25th birthday (provided they are unmarried or in full time education). In both cases the Iqama will not be renewed beyond their 25th birthday and its family visit visa time.