Social Security Benefits for Children after age 18.

I lost my husband in 2007. At the time our children were ages 18 and 13. My 18yr old was starting college in Sept. after my husband passed away in June. Since he was already 18 he didn't receive any social security benefits. I had to pay for his college out of my pocket. My other son was 13 so he did qualify for benefits. I also qualified until he turned 16. He is now fixing to be 18, but is a senior in High School so he will continue getting benefits until he graduates in May. I have saved every social security payment in his name for his college. I received a letter in the mail today from Social Security stating that I have to send them everything that I have saved including the interest it has made. Needless to say I am very upset. I am trying to figure out how the government can take the benefits away if you save them. What is the difference since I have paid for his living expenses, school expenses and everything else out of my pocket and saved everything he has received from social security. I guess I should have spent the social security checks and then just put my money in his savings account so they can not take it back.
If someone understands this please reply.
The whole social security system needs to be reconfigured.
When the child turns 16 and starts to drive the parents benefits stop. Just when you need it!
Then when they go to college it totally stops. Whats wrong with this picture????