I would start working on reframing your attitude towards the birth parents. Yes, I'm sure they're miserable excuses for human beings -- the problem is that you can't communicate that to their child. The best way to frame it (especially when the child is so young) has to be that they tried but just couldn't do it.
There are many adoptees here who can probably comment with more authority, but just as kids internalize the bad mouthing of one parent towards the other parent in a divorce, I would think that any animosity on your part towards the bioparents might be internalized by the child as some kind of taint that remains with him/her.
I'm sure you had no intention of openly deriding them, but even subconscious attitudes tend to come out, and the main goal has to be supporting a healthy sense of self in the child, and as hard as it is, that includes seeing some good in the biological parents that the child can take pride in (even if you have to fake it to a certain extent).
I have a friend who adopted 4 kids from the same mother (a couple of different fathers) and their problems are tragic. Sometimes she vents (outside their presence) about the damage the parents did (these children are older) but she also knows she has to try to give the kids some positive feelings about them, even if it's just what beautiful eyes they have, or that their extended biofamily must have been very smart to pass on such scholarly genes, etc. The last thing she wants is for them to feel that they come from NOTHING because that will make them feel like THEY are nothing.