Hi there
I am just back from taking my 11yo to Java for 2 weeks. Believe it or not, her favourite places was Bandung (we also went to Jakarta and Yogyakarta). We did things like visit a local school (I had met the school principal on-line a few years ago and we had stayed in touch), visit a volcano crater, swam in some volcanic hot water pools, went strawberry pickining, went to a children's angklung performance etc etc. In Jakarta she loved the Museum and in Yogyakarta, her favourite things were cruising around town in a becak and going to the Ramayana ballet.
Even though things had changed a lot since the last time I had been to Java (1996!), people are still really friendly especially to kids and there is so much to see and do. We were often approached by students volunteering to be our guides (no charge, just wanting to practise their English) at some tourist destination or other so this was a good way for my daughter to meet local young people.
The lowlights of the trip were daughter getting a tummy upset (we were out with an Indonesian friend who ordered a fresh fruit juice for my daughter and think it has ice mixed in with the fruit) and being hassled a lot by touts in touristy areas in and around Yogyakarta (this is a lot worse than last time I was in Yogya and made me think that I would probably stay further away from JL Malioboro if I was to go there again).
Unfortunately my daughter is not a reader like fivebs kids are but did get quite into Indonesian tv especially the soap operas (which are so over-acted that you really don't need to understand the language to work out what is going on). We also forked out extra to stay in places with pools and I found that particularly good in places like Jakarta where you do appreciate being able to cool down after a day tromping around in the heat and pollution.
We had previously taken her to Bali when she was 3yo and that was great too.
If you do decide to go to Java and would like some more specific information, just let me know. (My Bali information would probably be too old for you).
Happy travels!
Edited by: empressnasigoreng