No, they didn't forget--they didn't want to tackle this divisive issue while in the throes of battle against a foe they'd rather not fight and didn't want to risk being divided. There are many writings of the Founders cursing slavery and King George's refusal to allow the colonies to ban it. Study your history, bub, and maybe you wouldn't support such disgusting issues like legalized child pornography.
It wasn't that they forgot, it was that they could never have created a union if they prohibited it. Sometimes I wonder if it was worth the trade off.
The founders forgot to prohibit slavery. I don't think we should worry too much about what the founders thought per se.
U.S. Constitution, Article I, section 9, paragraph 1:
The Migration or Importation of Such Persons as any of the States now existing shall think proper to admit, shall not be prohibited by the Congress prior to the Year one thousand eight hundred and eight, ...
The importation of slaves was scheduled to be phased out starting in 1808. They didn't forget.