So, you chose the cheapest fare, which excludes certain benefits, and now you would like advice on how to obtain those benefits anyway, without paying more, just because you are traveling with your children?
If my circumstances required advance seat selection, my common sense would tell me to buy a fare that allows seat selection.
I don't know how it works specifically on China Airlines, but I imagine those who bought your type of fare will be given seat assignments at the gate. If you get to the gate early you could try to request seats together at that time and see if they can accommodate the request.
Also if I am reading their web site correctly it seems like the regular price to upgrade to Family Couch for 1 adult and 2 children should be $200, not $300, and there may be a promo price in effect for $120? But I think they would only have family couch on the SFO-TPE flight.
https://calec.china-airlines.com/FC_...y=us&locale=en
BTW you don't necessarily have to book Family Couch, just buy up to the next fare class that does provide advance seat selection.
If none of this is acceptable and you bought the tickets within the last 24 hours you should still be able to cancel entirely for a full refund per US DOT rules, see "Free Reservation Cancellation Within 24 Hours of Ticket Purchase" here:
https://www.china-airlines.com/au/en...ice-commitment