Misterioso this is probably far too easy for you but I thought i wouod mention it anyway. There exists a recorder tutor br Sarah Watts called "Two at a Time". It comes with a CD and teaches descant and treble together from the beginning. I have successfully used it with classes of top juniors who have a (very) basic knowledge of the descant. I have introduced the treble at this stage to improve motivation and get them playing in 2 parts. The book works well and the children love it.
In my private lessons I am sometimes asked to teach flute and then when parents realise the cost of a flute they back down. I then suggest treble recorder and this often takes off well. In this case I teach the descant after the treble is well underway. The descant is regarded very badly here as generations of school children suffered the torture of very bad recorder teaching in their class music lessons - until its use was removed from the curriculum as it was deemed to be counter productive. (I won't go into what other counter- productive elements replaced it!)
I have ignored that and have continued to teach descant on the grounds that it is the only instrument many French children will ever have had the opportuinity to learn - but I don't let it become anything like what it used to be in the hands of unqualified music teachers. My "window" for parents is the Christmas Celebration where many parents hear children playing descants properly for the first time in their lives.
Of course this is all now on hold until the sanitary restrictions are lifted and we can blow again.