For chair scoot and sit space, I'd follow the guidelines for a rug - 4' all around. Then you want to be able to navigate around the table while people are seated, so add another 18" all around. Are you locating the table at the end of the galley? Maybe you could use a banquette on one side or in the corner https://www.houzz.com/photos/3rd-street-bungalow-traditional-kitchen-austin-phvw-vp~214425

Carpets would be the preferred choice over tiles.

The chairs are a good match. Leather is probably best, great for kids' spills and accidents, but any fabric will do. Not sure on purple with green - those two colours together clash a bit for me. The green kitchen looks great, and I'm a fan of dark wood. The only thing throwing it off at the moment is the yellowish coloured walls. If they were repainted a soft white, this would make the green and the dark wood really pop, giving you a fresh and sophisticated, calm look which I think would work well. You could then upholster in a green to match the kitchen which would look appealing. Finish it off by some fresh white flowers as accents in the room to draw the eye and you've got a beautifully transformed space with a fabulous feature dining area :)

Thank you for your comments. I really appreciate your help. I think we are on the same track. The rugs I have brought home have fought with the terracotta colour plus the busyness of the small tiles. I did come across this picture on Houzz, which I think looks good. What do you think? It would need some of that burnt orange from the chair upholstery as well as the aubergine to tie in but I liked the dark border line, it matched with the grout.