I would do similar too. If you going behave silly like a toddler, then you be treated like a toddler and can sit in booster seat with tray on floor, in different corner of dining area from sibling, and facing away from other children. If you can use manner, and eat nicely, then you can join the group. But we not want hear lip smacking, slurping, burping, or see your food being chewed when we eating.
I would also up the grown up factor on the well behaved children. Make like game for restaurant. Have towel over arm like high class waiter, get little plastic stemmed glasses from dollar store for their milk/water, have napkin for their lap and go over top on nice eating manner at table.
If the sibling want play our restaurant game at meal time, they have behave nicely like if out for nice meal. If they want be foolish, then they can eat in toddler seat and away from audience including each other.