I have two cats, they are a year apart in age. When I got Sunkist (my youngest) Winnie (the oldest) was on regular food, but kitten food was what she wanted. I tried everything to get each cat to eat the food they were supposed to eat.
What I ended up doing was:
I fed Winnie her adult food in the kitchen. Winnie is a pig and she eats her food right away, so there wasn't that much left for Sunkist to eat.
Sunkist's food was put inside a closet with her water. I had the door open enough so Sunkist could fit in, but Winnie couldn't. I had to use weights so Winnie couldn't push the door open any wider.
This worked until they were roughly the same size, at which time Sunkist was older enough to switch to adult food. To this day, Winnie is interested in what food is in Sunkist's dish and Sunkist is interested in what is in Winnie's dish...which makes no sense to me as it's the exact same thing.
I was fortunate that I only had to figure out a way to keep Sunkist out of Winnie's dish, unlike your problem where each cat needs to stay out of the other's dish. Maybe by separating them into different rooms and making it hard for the older cat to get to the kitten's food, the older cat will go back to eating her own food...and the kitten will have to eat the kitten food that's in an area the older cat can't get to.
I tried quite a few things before I came up with this idea. Watching them eat to make sure they stayed out of each other's dish wasn't an option, as Sunkist is a grazer eating a mouthful every now and then and Winnie just inhales the stuff.
Hope this helps.