In reply to MisterPiggy:
Thanks. Let me know how they go for you.
Though we don't yet know my educated guess is that their fingers will adapt positively. They will look unusual to the general population, but will actually be suited to the demands placed on them.
This shouldn't mean they automatically develop issues when they get older, in fact it may be the opposite as their adapted, robust, fingers may well be less injury prone. We just don't know, yet....
As with all sports and growing bodies variety and not doing too much are the important factors. Don't worry about letting your nieces and nephews have fun at the wall.