Ethics of in-app payment in educational apps

As a relative newcomer to monetisation in education I do find this fascinating. Teaching was always a vocation for me and it was a great bonus that I got paid to do the thing I loved. Now my energy had moved to the educational potential of mobile devices I find the bills will still need to be paid. So can we find an ethical balance?
In App payment 
We are working on ios so I don't have to worry so much about accidental purchase (so long as parents have fixed settings see previous post). But still it does not feel totally right, people will think they are downloading a full app and I know I feel a bit cheated when this pops up (often when I have already paid out a bit!).
Having read around, the bonus is that you rank higher as it looks free and involves only one download, but its a big download as your effectively downloading two apps. 
Free with ads
Come on they are kids, I hate my kids being sold to! but it is tempting to have educational ad's on the 'A' level range as could help older students (16-18) identify other resources. 
Free with ad's until in-app purchase,
Supposed to be the max dosh option but again not keen on ad's to young children.

Lite version
Its good and honest! People will get to see what they are paying for and make an informed choice. Does mean two downloads which will put some off but seems the best option for now. 

Dinosaur Letters Lite

Luckily our apps do not involve analytics much further than how long sectional are played, and if it crashes, ill leave that can of worms for others.