Stanza 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Before it cloud, Christ, lord, and sour with sinning,

Our speaker is urging Christ to get hold of this bounty and lushness before it spoils, before it goes bad. The way it goes bad, we learn, is through sin.
(That's what happened with Eden. Forbidden fruit, original sin…)
The syntax is getting kind of jumbled. This seems to reflect an emotional turmoil.
All of a sudden a sense of anguish has entered the poem. We say anguish because there's a strong sense of urgency ("– Have, get, before it," from line 11) and pain (the hard c-sounds, and the way he keeps repeating and rephrasing – "before it cloy, / Before it cloud, Christ, lord, and sour with sinning").
When we get to "Christ, lord," not only does it become quite clear that our speaker is Christian, but the poem also begins to sound more directly like a prayer.

Innocent mind and Mayday in girl and boy,

It's the innocent minds that are threatened with the possibility of cloying and clouding and souring with sinning. So if we put it together with the line before, our speaker is asking Christ to save the innocent from sin.
The way the line before this one works, it seems to suggest spoilage in both directions: both of the Eden-like natural world and of the innocent children's minds.
Should we understand that the sweetness is what leads the innocent mind to sin?
It sounds pretty inevitable, kind of like the change of seasons.
There's definitely a lot of complicated emotion going on here. There's that sense of urgency and pleading, combined with a feeling that it's going to happen anyway – the kids will grow up and they won't be innocent anymore. They'll be jaded, spike their hair, and listen to punk rock.
We had those lambs racing into the poem with their innocence, and now we have that inevitable loss of innocence.

Most, O maid's child, thy choice and worthy the winning.

"Maid's child" is probably referring to Jesus, born of the Virgin Mary (one of the meanings of "maid" is virgin).
So here's one way you could read line 14: "Jesus, it's up to you – won't you win over these innocent children, and save them from sin? It would be a very worthy thing to do, to win them to your goodness (and keep them free from sin)."
The syntax gets pretty confusing again.
The poem has definitely turned into a prayer. Though, honestly, it could have been one all along, just one that changes gears a couple times, from awe and praise, to anguish, despair, pleading, and finally…
This last line seems to acknowledge, as most prayers do, that the power (the "choice") is in the hands of God.
"Choice" definitely brings up a lot of questions and possibilities. What does it mean that it is God's choice to have or to allow sin?
The word "choice" also might brings up the idea of free will, and maybe that's the answer to the question we just asked: in order to allow free will, God has to allow sin.
Do there seem to be a lot of subtleties and multiple ways to read each line? A lot of things hinted at and no clear answer for how to understand everything? Yes, that's about right.