What sort of thing do you want? In Chippenham you've got the usual 'child-friendly' pubs such as the Brewer's Fayre or there's another one I can't remember the name of. I think both have small play areas. Not great food though and you run the risk of coinciding with birthday parties.
Or there's a Pizza Hut and a Pizza Express (the latter always seems clean and staff are friendly).
Or how about the Dutch Cottage Tearooms - this is a really lovely family-owned teashop. They do light lunches until 2.30 and have a morning and afternoon tea menu which is available all day - cakes, scones, toasted teacakes, 'things' on toast etc. I think I've seen highchairs in there, but I'm not sure, you could ring and ask.
Or there are lots of pubs which do food, both in Chippenham and surrounding villages.
Or how about Whitehall Garden Centre - this is about 2 miles from Chippenham. It has a restaurant which closes at 5 p.m. Lovely home-made food, using lots of local ingredients. I don't know what meals are served after the lunchtime slot, it might be more sandwiches and cakes, depending how 'late' you mean on Friday afternoon.
It has highchairs and is good for a break - things for children and adults to look at (fish, small pets, food hall etc), and an outside play area if your children need to let off steam!