Depending on what age your kids in your house are, that will determine whether they are 'factored' into the assessment. A child is no longer classed as a 'child' when they become 19, however there are variations on this, their circumstances, ie working, claiming benefits etc. When someone makes an aplication to the CSA, the effective date, ie the day the case becomes 'live', is the day when YOU are notified of the application, NOT when the person makes the application. Request a DPA, and see what info is contained within that, then we'll see how to proceed. Is there no way of making a private arrangement with your ex for an agreed payment amount, as the CSA are extremely keen on this as it saves them time and money. It costs a fortune to run this Agency, hence the new system that will emerge next year, when the charging will begin, if the system works, which is extremely doubtful.
Regarding the DEO, if they do issue one, go through it with a fine tooth-comb, as if it isn't EXACTLY spot-on, ie, if the company is called Fish & Chips Limited, and they address it to Fish & Chips Ltd, then it isn't legal. Also, you should receive sai copy of DEO for your files as well.