Not an easy task. I am just finishing up with the child's rocker that Norm
did a few years ago.
I assume that the legs come out at angle to the seat. The problem is to
determine what the side too side and front to back angles are and then work
with tipping the drill press accordingly. You may also need some wood shims
cut at angle if you have to do a compound angle, which would occur if the
legs come of the seat at different angles.
Do you have the original legs to use as a guide?