We took our kids to Loch Fyne last Saturday at 6pm; they were the only children in there but that was fine, it was about a third full and very relaxed. There is a kids menu at £6 for smaller portions of some dishes plus ice cream; my 5 year old enjoyed haddock goujons and chips, while the 8 year old moaned about only having a kids portion but got a huge bowl of moules and was more than happy. We enjoyed our food too, not cheap but good quality ingredients.
They serve food most of the day I think, including weekend breakfasts. On the basis of our first visit I think Loch Fyne will be a welcome addition to the restaurant scene in Sheffield