Which ever one of you claims the family benefit gets the maintenance off of the other one. That is something they will decide upon in court. It often goes in the mother's favour tbh. Bear in mind though that if it is you who is to receive maintenance, the figure will reduce by 1/7th for every night they are at their dad's. Check out the online CSA calculator (google it) to see what the figures are. If they are with their dad 50% of the time you would fall in to the '175 days or over' category.
If you look on the calculator here there is a drop down option that suggests the amount of days per year the non resident parent (in your case which ever of you doesnt claim the family benefit) has the child or children. With a 50/50 split, your ex will have the children 182.5 nights a year (half of 365) So he will fall in to the 175 days or more category.
They do this so that the non resident parent isn't paying for the child/ren on days when the children are actually with them. Otherwise they are paying for the child twice, in effect.
You can arrange between the 2 of you that if there is 50/50 shared care, neither of you pays maintenance, but that is between you. Legally, the CSA rules apply.
In court they may apply something else, particularly if your ex or yourself have a higher income than the other. But be wary, this is only enforceable for 12 months, then it can, if either of you wish, revert to the CSA rules quite simply.
They don't often explain that so you start to rely on the amount that the court rule, only to find yourself skint 12 months later.