My boys are 7 and 4, and little sis is 2.5. They can all be goofy and exhibit bad manners, poke each other with forks, try to steal food from each other's plates, crawl under the table and pinch each other or poke Dad in the nads, etc. It can get really frustrating! Here's what I do to help remedy the situation:
1) The little ones are allowed healthy snacks when our 4yo gets home from preschool at 12:45, then when our 7yo gets off the school bus at 4:00, they have some fruit and a big glass of water. That seems to chill him out and satisfy his appetite enough that he can do homework, but he burns through it before dinner.
2) While I cook dinner, the kids are allowed to watch TV (something at least semi-educational, like a science/space show, Team Umizoomi, Sesame Street, Super Why, etc.)
3) Dinner time starts when I tell them to turn off TV and come set the table. The 4yo puts out placemats, the 2yo does napkins, and the 7yo does silverware. Once we sit down to eat, the expectation is that everyone sits up straight and uses their manners, and they must eat at least some of everything on their plate before they can ask for seconds. If they act rude, potty-talk, or get up from the table, I warn them once, then their plate gets taken away. They can earn a second chance if they apologize and behave nicely, but if they repeat the misbehavior, they're done. (doesn't take too many times before they figure it out!) When they finish their dinner, they say "Thank you for dinner, Mom & Dad, may I be excused?" and then the 7 & 4yo are expected to clear their own plates and pick up any mess they left on the table or floor.
Most of the time, this works pretty well. If you establish a routine where you clearly communicate expectations and consequences before hand, it is easier to enforce, especially when they understand the reason is to have a nice family dinner together, where everyone is happy. I also make sure they are hungry before dinner, and what I feed them is mostly stuff they like to eat...I do try to expand their horizons occasionally, in that case I tell them they just have to try a bite to see if they like it. If not, I say "Well, maybe you haven't grown those taste buds yet - we'll try again later." Then they feel like trying new foods means they're growing up!
Good luck - I know how challenging it can be, but just find a routine that works for you and stick to it. You'll be fine!