My understanding is that yes, you do have to declare all of your income(s) regardless of their source.
On the form E, there is a section where you can also declare all of your outgoings (including reasonable amounts for holidays, days out with the children etc)
If there is a gap between your income and outgoings and your ex has the ability to pay it, then it could well be the case that he should give you child maintenance, regardless of you both having them 50:50.
The welfare of the children come first, and it wouldn't be fair on them if they are experience a vastly different standard of living in each of their parents' homes.
Thank you for your reply. Much appreciated. Yes, there is a difference in standards, he can acord holiday every year whereas I cannot and more other things. I was not sure if it is right for me to think I should have some spare income too.