Considering doing a $6 for the 6 year old (in lieu of gifts)... how would I put that on an invite?

I would just ask family to do the $6 thing since you will have enough if they all pitch in and leave the other people who you don't know so well like classmates to bring whatever they want. Of course if one of the moms calls and ask you straight out you could tell her about the 6 bucks.