I don't think there's any definitive answer on this. But my sense is that it'd be permissible to do this only if your child were younger and still eating baby foods. At 21mo, they're basically able to eat some of what adults eat, such as rice, noodles, sausages, karaage, hamburg and fruits. Most casual restaurants will have kids' meal sets. While kids' menu items are geared towards kids older than yours, nonetheless your child is already in the kids' menu age category. If there isn't a kid's menu, then I still think that the staff would frown upon a child taking up a seat and eating bento bought outside. Based on these considerations and sort of having a sense for Jpn's unwritten rules, I personally wouldn't do it. We've gone through this with our own kids at various ages, and I don't think we've done anything like that, except at food courts and perhaps at restaurants inside hotels where we were staying at. When our kids were still babies eating baby foods, I believe we were doing this without issues, and I can't imagine restaurant staff not being okay with that. But outside bento and snacks would be very different than baby foods. Now, I could be wrong on this. You could try... but if you're going to try it, then you should definitely ask the staff first if they're okay with it.

Also, with a 21mo, I think inevitably some places are going to have to be off the table. I have kids 7 and 6 and it wasn't until recently that we were able to start eating at places like little sushi and ramen places with counter seating.