Actually he can't, and no...he was not on kage level as a child.
Maybe he could block a lesser Biju-Dama with his Susano'o, but the Juubi's BD I doubt it.
But this isn't a reason to why he's weaker then Minato. Their technics are completely different...
Itachi is very OP in 1x1 against most opponents if he can finish the fight fast. Minato is to very OP in 1x1 fight but doesn't necesary need to finish his opponent fast, because he has enough stamina to keep up.
Besides he has more utility then Itachi on a team fight. Sure, Itachi can be very usefull with his Susano'o and Amaterasu, but you know what happens if he uses it more then some minutes.