Did the founders of philosophy think that children were mindless creatures who did not deserve equal rights as their adult counterparts did ?

Well first philosophy does not make or give rights, it would merely discuss rights as a object of discussion,

If you are talking about the founders of laws for a specific nation, no they understand they are not able to have mature thoughts and deserve the right to be protected. Children have special rights and protections, not equal.

It is my understanding that children were thought of as "small adults" even through the Victorian Age!

It wasn't until after the age of psychology and research in related fields that some understanding of child emotional and personality and neurological development were understood at all!

In addition, children were thought of as the possessions of the parents, not to be interfered with.

Today, we have a more enlightened view... we understand that abuse or neglect in the first six years of life scar a child. Other faulty parenting makes for a miserable child and then adult.

As for legal rights in America, there have been civil rights for children for quite some time protecting them from sexual abuse and neglect, as well as physical and mental abuse. I don't know when these laws were instituted, sorry.

If you want the question to go another direction, lets go for it, I like some of the more "idea" minded questions, getting tired of 13 year old not knowing why it is wrong to date a 30 year old type questions.