There's a great, kid friendly restaurant called "Giraffe" in Islington at 29-31 Essex Rd. It's really excellent and worth the small trip. They open at 9am on Saturday (just rang and checked). Here's how to get there:
Go on the southbound Northern Line to Angel. It's one stop from King's Cross -- the signs for the southbound trains will say "to Old Street, London Bridge and something else".
At Angel, exit the tube station and head straight. It's a slight right, but as long as you don't turn left, you're ok.
Stay on your side of the street and keep walking past where the road splits into Upper Street and Essex Road. You will be on Essex Road.
After you've passed Tesco Express on your right, cross at the next zebra crossing and keep heading in the same direction.
Giraffe is on your left-- it's orange.
They have fruit smoothies, great pancakes and full English breakfasts, smoked salmon with scrambled eggs on toast and are seriously the most kid friendly place ever.
If you would jog right at Duncan St (by the York Pub about a block up from the tube stop) and head up Islington High Street, then through the pedestrian street, you'll walk through Camden Passage which has an antiques market and a toy store. You'll end up at Tesco Express. (I had a link to a map, but it wasn't working and VERY long, so I deleted it. You can look at streetmap.co.uk and search for Essex Road, N1 to see the area)
It's a little out of your way, but a great place.