It's common for those with absolute pitch to have more trouble with "black keys" than "white keys."
Similarly, play them pieces in these keys. Something like Chopin's "Butterfly" etude, or "Raindrop," will provide them concrete examples of these keys/pitches.
If the child is already fluent on an instrument, having them play things in keys like D-flat and G-flat will certainly help develop that pitch memory.
If the child is learning an instrument and is only up to keys with, say, one or two accidentals, I recommend letting the learning process happen naturally. In other words, let them get to pieces in D-flat when they're ready; don't push them into D-flat too quickly just in the hopes it will improve their pitch memory.