I thought Ft William/Glencoe was great for the kids, as Caberfeidh mentioned. We took our boys there a few years ago, they were 8 and 10, and we had a few great and mildly scrambly walks.
We went up the gondola and hiked around up there, took them to Nevis Gorge/Steall Falls and the wire bridge, and up into Coire Gabhail/Lost Valley and they loved playing on the boulders.
Check those out and see if they're the sort of thing your kids would be up for- mine did fine at that age, but there's nothing they love more than a nice bit of rock... wonder where they get that from