I started AAS 1 with my 10 & 9 yos together.
I have explained to them that it might "appear" to be too easy, but that it is laying a foundation, and I don't expect it to take them more than few weeks. I have modified it quite extensively, as we went through the phonograms once (only reviewing vowels & ch) they now most of the other rules, so I just kind of wing it, look at some parts I think are important and emphasize them and away we go! We had only one 2 days worth of lessons (1-4) and yesterday I dictated 50 words (yes they were all 3 letter words) with nary a single mistake.
They felt that they had accomplished something, I was happy that they didn't screw up such easy stuff, then I let them put stickers through lesson 8, after going over whatever rules needed to be covered.
Even though it is totally scripted you shouldn't be afraid of modifying it when you need to. We only did the tiles once so far, I can't imagine using them more than once before we move on to harder parts. There can be an advantage to using it with an older child, as you don't have to repeat the tedious parts until they actually don't know it.
anyhoo, my 2 cents. I am looking forward to level 2, when i can find it cheap enough!