They want to sit on your lap
Lap eating can be the bridge between a snack and a chair. Offer the familiar food there, with the plate where you can see it.
A lap can still be a meal
Move to the chair when you can, without making the move a fight. Some meals will be lap meals. That is still eating.
Keep the plate where you can see the food go in. A lap is not a reason to stop watching them chew.
The familiar food first
Start with the food they accept. A new food on a lap, with no table, is harder to manage and harder to see.
If they are willing to move to a chair for the same food, try it once, then drop it if it becomes a wrestle.
Safety on a lap
Hot soup and a wriggling child are a poor pair. Lukewarm food, small pieces, you watching.
Whole grapes, nuts, and hard raw vegetables stay off, on a lap or a chair.
A sentence you can say
“You can sit here. The rice is where I can see it.”