Not too soft, they have a kinda slick silicon feeling to them. They are cupped on both top and bottom, which means you would roll your blu-tack into balls and press into the cups.
I don't have any Blu-tack on hand, but just now tried some of the cord weatherstripping rope caulk on the Iso's rubber feet, as it is very similar. Didn't stick too well at all, so based on that I wouldn't recommend depending on the blu tack to make the stands kid proof. Maybe someone else has had better results with actually using the blu-tack.