I told my 4 yrs old that we just gave each other a special kiss that only a mummmy and daddy can give to each other...she was happy with that

told my 7yrs old that mummys and daddys fit together like a puzzle when we want to have a baby he got the idea

the next question you will get is how does the baby come out?
Those were the two things I was going to suggest. I've heard "special hugs" suggested as well.