He who respects the Infant's faith Triumphs over Hell & Death.
This couplet is the opposite of the two that came before it. Those were talking about the negative consequences of non-belief, but this one's praising faith.
It's not that Blake thinks that unquestioningly believing something is totally great. It's more that he views faith as something essential to human beings. If someone tramples on an infant's faith, they're crushing something fundamentally human. But if someone respects it, then that person is headed in the right direction.
They "triumph over hell and death" because death isn't a genuine reality for them anymore (and, in Blake's view, hell is just a state of mind). They've put their faith in this eternal order, this eternal reality.
Also, this couplet repeats the same odd rhyme from lines 85-86.
The Child's Toys & the Old Man's Reasons Are the Fruits of the Two seasons.
Kids like to play with toys and older people like to play with ideas (though it's fair to say that that's not always true)—that's what Blake's saying. But he's also drawing an analogy between the two. Old philosophers fiddle around with ideas the same way kids fiddle around with toys—you move from messing around with the physical world to messing around with the intellectual world.
The toys and reasons are, metaphorically, "the Fruits of the Two seasons" because they're the ways people enjoy themselves in those two time periods.