The Background:
You are young and have just moved into town. One night, a stranger with astonishing eyes sweeps you off your feet. The next morning, he is gone, a note left in his stead. It reads:
I had to leave to keep you both safe. I am sorry. My DNA has been enhanced, I was recruited to be a superior soldier. Last night, I utilised my skills to flee but I can not stay. The military is looking. Live well and take care of our child, keep him safe, keep him hidden.
You raise the child, tutoring him at home and soon meet a new man with whom you start a family. His work forces him to leave for long periods of time and you struggle to juggle the children running underfoot and keeping your secret from them and the world.
18 years later:
There is a letter in the mail yet the stamp is absent. A single sentence is printed on a sheet of white paper; it is over, he is safe. Delighted, you run to the son who has been hidden for his entire life. His life and his freedom await.

Startup:
-Make a female sim in CAS (family orientated traits, logic and a home money earning trait, e.g. for fishing, gardening or painting is advisable). Set the life time wish to raise 5 children from babies to teenagers.
-If you want, make the two male sims in CAS too which will father the children.
-Build a house with a room in either the basement or the roof where the hidden child will have all he needs (toys, bathroom, bed etc.).

Rules:
-No sim can be in the basement with the child except the original female sim and toddlers or younger (as they won't remember).
-No progression towards skills that you will be using for the challenge e.g. logic may be made until the sim has conceived the first child.
-The first child cannot leave their room at any time until they become a young adult (lock the door to allow only the mother entry).
-Once the first child is born, find a partner as quickly as possible (you are not allowed to leave the house until the house is a child) and conceive a second child.
-You must have a total of 5 children, in as quick a succession as possible, the baby must be born before its older sibling becomes a child.
-The father can only come over when you are trying to conceive and may not be asked to stay or to leave.
-You may not hire a babysitter, other services are allowed provided they do not need to enter the hidden child's room. E.g. You can not call a repairmen to repair the child's broken shower.
-You must tutor the child, he can not go to school.
-The hidden child cannot contribute towards household funds, but other children can.

The aim:
A) Raise all children and complete the lifetime wish of the mother
Raise the hidden child to adulthood and allow him to find his own way in the world (must move him out of the house with earnt money)

That's the challenge. Would love to hear some opinions on it, I just trialed it myself and the hidden child has become an adult. I was able to pick all his traits and he earned a B and A as a child and teen, respectively. There were periods that had an D and once an F. He had mastered the logic skill and was partway to completing his life time wish (master of body and mind).

If you decide to play the challenge, good luck and have fun.