When love bears fruit and children arrive, the relationship is put to the test. The woman gives her love and care to the child. She does not have much surplus for her partner. They can no longer go to the cinema or out on the town. Life is not like it used to be when it was only the two of them on their own. There is the children demanding as good as all they've got.
Even when, the kids grow older and more independent and no longer require full attention from their parents, they are still there. You cannot have sex on the kitchen floor when you feel like it. Sex at night can turn out to be a longer exercise of planning - they can hear you or they can come blowsing any time.
When they are teenagers they certainly do not sleep at night - cause they've turned day and night upside down. Then you need a padlock on the bedroom door.
In short, children in the relationship give challenges regarding the sex life. It's all too easy to forget each other in the turmoil of children. You need to remember to make a space where you can be together as lovers. This is very important if the relationship is meant to last.
If you are finding it difficult to manage to develop your relationship in the midst of the children-turmoil it can be beneficial to see a couple's therapist.