Wedding postponed for 2 years. Flowers girls will be 9 y/o

Technically, there can be no age limit because it's just a ceremonial/decorative role with no real function. But since they will be old enough to have their own opinions, I would wait until a couple months before your new wedding date and just ask them what they want to do. You could give them a couple options that you like and just let them choose.

My brother and my SIL had my 13 year old sister as their "flower girl"/jr bridesmaid. Theres also a huge trend now of having adults take these roles. I was just in a wedding last year where they had dogs and their owners fill the ring bearer and flower girl role. Literally just do whatever you want unless the child herself no longer wants to participate.

9 is old enough to have an opinion. Ask them if they'd like to still do it. If they say no, i'd offer to let them process like part of the bridal party and stand after your BM's or sit in the front row. IF they say yes, then cool.