I suggest the are two ways of looking at this requirement. Through this board and contacts, option two could be explored.
ONE
The light weight pistols will not be much use for serious 10m shooting, however, they are very good for junior shooters to introduce them to the sport and shoot adequately at 7m (Pony Club et al) and 6 yards. These are generally CO2 cylinder units the triggers are a bit agricultural but grips surprisingly good. You will probably sell the unit for a decent price as the Pony Club is getting more serious on the shooting side should you want to upgrade. Shooting two handed will assist the hold but not adequately address the core skills required when they are older.
TWO
In Scandinavian and other countries, a counter weight system is used that cradles the underside of the pistol cylinder and over a pulley wheel to a counterweight (I wish I had a picture of one).This enables more competitive pistols to be used so that sighting and trigger are primary skills and correctly developed. I recall at 10-14 flailing about holding the air pistol and snatching the shot off as the sights may have looked OK a few moments ago, (some say this continues today !) As the juniors get older, the counter weight is gradually reduced, in your case, different weights can be used to help develop the muscle sets but at a sensible rate for your three. I do not believe any UK competitions permit this assist for junior shooters but have seen it provided at some serious matches in Germany and Finland. Accordingly, you may want to get a light weight trainer but create a cradle to help them learn?