Just east of the Tate Modern, there is a Nandos in a railway arch. It's before London Bridge.
By chance, when we were there yesterday, we got table 38, which is right next to the water. You're under the bridge, with a great view of the boats, and out of the way of the other diners. It's a hassle to get to with pushchairs (up some stairs) but thoroughly worthwhile. Tables 38 and 39 are great. There are some other ones in an arch, further into the restaurant, but the window doesn't go all the way down to the floor, so the view isn't as good for little ones.