But best of all was the warm thick slobber Of frogspawn that grew like clotted water (8–9)
The only way to get frogspawn is if two frogs mate. Our speaker might not be thinking about this yet, but those are the (slimy) facts.
The daddy frog was called the bullfrog And how he croaked and how the mammy frog (16–17)
The male frog courts the female frog by croaking. Love is in the air! It's part of their mating ritual. Again, it doesn't seem like the speaker has made this connection yet, but it shows he is learning about the sexual reproduction of frogs.
Laid hundreds of little eggs and this was Frogspawn. (18–19)
Again, there's only one way to make frog babies: two frogs have to mate.
To a coarse croaking that I had not heard before. The air was thick with a bass chorus. (25–26)
The frogs' croaking is part of showing off for female companions. Is there anything hotter than a baritone croak? Not for lady frogs, there isn't. The fact that the speaker hadn't heard it before might be Heaney's sly way of saying that he's naïve when it comes to sex (both about the frogs, and potentially about humans, too).
Right down the dam gross-bellied frogs were cocked On sods; their loose necks pulsed like sails. Some hopped (27–28)
Again, this is the bullfrogs' display. To get in good with the ladies, they slap around and make a lot of noise. The boy, however, is disgusted by the show and makes a break for it. Is this when he makes the connection between the gross display and sex? Who knows. Regardless, this show makes a profound impact on him.