You can upgrade your house to "XL" the day after you get married (as long as the day after your wedding is not sunday, because Gannon does not work on sundays)
The kids bed will be available to be bought at Chen's shop, exactly one (1) season after you're married. i
married Vaughn on the 4th of fall, the kid's bed was on sale the 4th of winter. and the day
after i bought the bed, 'the big news' event happened when i woke up. thats the event
where they tell u(or ur wife if ur playing as a boy) that u/their pregnant.
Also, your baby will be born two (2) seasons after 'the big news' event, on the first non-event
day. if a festival falls on the day your kid was sposta be born, then your kid will be
born the day after the festival.
hope this helps you :)