Clive Hart suggested that Books One and Three display some mirror symmetry, with Book Two between them. It's sometimes called the book of the children, or, via Vico, the book of heroes.
The early vignettes of Tristan and Isolde, and Mamalujo, ended up in II.4. The Muddest Thick provided an early fulcrum for II.2.
II.1's theme of children's games is certainly universal, going back past kittens play-hunting and play-fighting. Joyce relies on Norman Douglas's "London Street Games"  for phrases and themes.
We should expect the children's games to rehearse the interactions of the adults: a temptation, a sin, a cad, a battle, a fall, a river to wash things clean. The main game that Joyce chooses has Shem trying to guess the color of Issy's underwear, with Shaun's chastity as counterpoint. There are surely biographical echoes of 22yo Joyce courting 20yo Nora.
This was conceived after the Muddest Thick's scene of Shem shocking Shaun with a diagram of their mother's bottom, framing both II.1 and II.2 as the children's evening activities. A likely biographical echo imagines Joyce with his puritanical 3yrs-younger brother Stanislaus.