Of course it's okay to leave yourself out (if that's what you really want) but are you not, in fact, a biological family member of the mom and, thus, the child?
The mother is your aunt, if I remember correctly. That would make you a cousin to the child (biologically speaking). If the family tree is not THAT large -- to include extended family -- then go ahead and leave yourself off IF you want to. Otherwise, I'd suggest putting yourselves on where you would 'fit' biologically.
I assume this is a gift for the child as a link to his/her PAST -- the family of nature. I'm not trying to be cold about it but I would have given ANYTHING to have something like that from my natural family as a child. In fact, I am making one for myself now. And, no, my a-parents wouldn't belong on it as THEY (unlike you) are NOT biologically related to me in any way.
Just so you know...the "family tree" thing, especially when it comes up in school settings, really really sucks for an adopted child -- well, except for those "I've accepted my adoptive family as my own and I just use them" adoptees, which I am not one of. Even for them, it makes the 'genetics' lesson take a giant 'dump' (pardon the expression) when using the a-family because there is NO explanation as to "where the brown eyes came from" or some such.
Do what you think is best. If it's a gift of heritage, leave yourselves off or put yourselves on in the position you belong -- biologically.