If the bellows is original on the Isolette then it will leak. If 'everything works' then the shutter, focussing and aperture should have been dismantled and cleaned, and the bellows must have been replaced with a modern set. Nevertheless, the first time the camera is picked up by the bellows it will be destroyed. An Isoly might be a better choice, as it is a ready-to-use design with no bellows or separate shutter-cocking required. It also has a large viewfinder and click-stop, pictogram-based focussing as well as multi-exposure prevention.
For small hands, an Agfa Clack is very awkward to hold completely steady during exposure (tried it with my nephew). Lubitels can have the correct coupling between the taking and focussing lens easily jump/move by a few 'teeth', which then makes it impossible to focus correctly. An alternative might have been a Flexaret, which would be have an automatic frame counter and be serviced, if bought from 'Cupog' or similar. The price would probably still be a bit high for the intended purpose though.