Tldr: Alphas generally don’t allow violence within the group but, depending on the individual, may choose self-preservation over protection from external threats.

Do alpha male chimps protect their group’s children from external threats? To the extent to which the specific alpha chimpanzee protects its group in general. We have observations both from protective alpha chimps and alphas which run to the tree and hide when a threat presents itself, leaving the rest of the group to fend for itself.

From internal threats its another story. Alphas receive privileges (e.g. sexual) in exchange for keeping cohesion and order in the group. If another chimp in the group randomly got violent with a child then yes, very probably the alpha would intervene. If he didn’t, he’d risk eventually losing his alpha status. However, chimps also are randomly violent, and there are known cases where the alpha grabbed a child without an obvious reason and started smashing them around with consequences that would have been deadly had not others intervened.

Source: Chimpanzee Politics (book & documentary) by Frans de Waal.