It's bad to hurt a parent's feelings.
Ken wants to talk to his mom about a present for his dad.
Ken doesn't want his dad to hear about his gift.
Ken tells his dad that he needs mom's help with homework.
Ken tells his dad he doesn't want to talk to him when he needs to talk to just mom.
Now dad won't want to listen anyway and the secret will stay hidden.
Ken hurts his dad's feelings since he doesn't know the reason.