The BHF course is graded for kiddies learning - as you say even little children can use a phone in an emergency and could put someone into recovery position if shown the way to do it. If you look at the Ordinary Day DVD, it shows on the back the suggested ages for each section - if you don't have the DVD let me know and I will mail it to you.
Having said that, some of my teams have taught BLS to Rainbows, Brownies, Beavers and Cubs as well as the older end of the Scouting movement.
I am very supportive of the move to get BLS onto the National Schools Curriculum. I feel that it is important to catch them young, so go ahead. There are leaflets for the teen years and a good booklet called Emergency Life Support for Everyone, which I often give out.
Got the above back from my BHF contact, I hope it helps.