Kids have a peer for company. Lots of individual attention. Almost constant activity for both players. Competing with the lesson partner facilitates intensive singles instruction. The intensive, uninterrupted work needed to fix a stubborn stroking problem can be hard to attain.
Some younger kids feel more comfortable with more kids around.
If one kid learns much faster than the other, the other can become frustrated. Kids who want to learn quickly, but feel more comfortable with a friend around. Players who should compete against a peer and are not advanced enough to practice competing against the pro. Players who are getting involved in team and tournament competition. Families who cannot afford private lessons.