Bulbapedia says that moves with a charging turn only hit once:Every attack that Mega Kangaskhan uses will hit twice, with the second hit being half as strong, thanks to Parental Bond. This includes moves requiring a turn to charge and Hyper Beam/Giga Impact. (it will still take just the next turn to recharge)
Moves with a charging turn only strike once, even if the Pokémon becomes fully charged in one turn (such as with a Power Herb).