Can I expand my polycule after I marry someone? A child much be useful labor but I also want to experience everyone's hangout events
yes!! you absolutely can!!
you can date anyone, even if you are in a union! more than that, you can still experience all the flower events even if you turn someone down - they'll just be more platonic hangouts, but still have the emotional closeness and character lore!! you can get full flowers with them, see all the cutscenes, dating or not!!
it's one of my favorite parts of the game - no jealousy issues, you can just be poly!! unfortunately you can't be in a union with more than one person, but you can be dating however many you like!!
quick note: children don't provide a lot of extra help, but they are some of the best developed and designed children ive seen in games like these. i won't go into much, bc spoilers, but i will say ive had some legitimately emotional moments with my kids in-game, which is a huge departure from the 'small sprite that walks around your house/farm' model ive seen in other games. children are worth having for the experience alone!
more information on extra farm help goes into spoilers, so:
spoilers for late game farm mechanics under the cut