I am not aware of an article discussing untoward effects of gait plates on the midtarsal joint of children. I believe that this concern emerges from time to time because of a false perception that gait plates work by pronating the midfoot (read midtarsal joint). This is incorrect. If you go back to Schuster's original article on the gait plate extension it is designed to alter the break of the ball of the foot in propulsion requiring external rotation from the hip to complete the step. This is why they must be made of a (relatively) rigid materil and why they must be used in a shoe or sneaker that is flexible in the ball allowing the extension to exert its effect. Outer sole wedges (essentially forefoot valgus posts on shoes) will, on the other hand, pronate the midtarsal joint. I teach that one of the beauties of gait extensions is that they can be added to controlling foot orthoses with the orthosis working in contact and midstance and the gait extension exerting its effect after heel lift. As you know, so many of these torsional problems are accompanied by pronated, compensated feet.
As to your second question, the only outcomes study on gait plates I know of is the one by Tony Redmond in Foot and Ankle that presents measureable improvement in the gait angle with the use of gait plates. I know of none that discuss any perceived impact on the midtarsal joint.
Finally, as to pros and cons. The pros are that they can help the angle of gait and reduce tripping and falling particularly in an age group too old for casts and splints. The principal con is that they are of only limited effect and will, at best, effect some angular improvement but will not convert the gait completely or normalize any underlying torsions.
Professor and Chair
Department of Pediatrics