That's simply not true. It wasn't until the 26th amendment, in 1971 that the right to vote was set as 18 in the US. Before that even that right was dependant on state based laws. (Note that under 18's don't have the right to vote at all)
OTOH, Tinker v. Des Moines Independent Community School District[1] holds that children DO have the right to freedom of expression.
http://people.howstuffworks.com/do-children-teenagers-have-c... is a pretty reasonable discussion
[1] http://en.wikipedia.org/wiki/Tinker_v._Des_Moines_Independen...
If a "right" can be signed away, it's not really a right. I recall reading about a case where a woman agreed, in writing, to waive her right to maternity leave under FMLA. She then changed her mind, and the employer sued and lost on the grounds that rights cannot be signed away.
Looked at from that lens, you're arguing that a child's rights can be signed away. My argument is that therefore they aren't rights.
Thus in most of the world today, you cannot willingly sell yourself into slavery.
But many of these rights (eg, the right to carry arms, the right to enter into contracts, etc) are limited to adults.