For both Karate and Ballet you'll need a Community Centre:
Alternatively you can 'buy' a "Record Player" for your ballet aspiring pre-teen for either 7 NBRs or 25 LP. The Karate "Sparring Dummy" costs 8 NBRs or 20 LPs.
Ballet: Can be done at the ground floor of the community centre. No collectibles while practising, gives only level experience and XP points, but unlocks various hobby-related items while completing levels. For at home, a "Record Player" like at the community centre is available from the home-store under 'NBR'/Social. It needs either 7 'neighbours' or a payment of 25 LP.
BUG: the pre-teen will not change into the ballet outfit when practising at home.
Pros: unlocks ballet themed items/wallpapers when levelling up
1. All Left Feet (white outfit)
2. Novice (pink tutu for girls)
5. Expert (black tutu for girls)
6. Professional (black tutu for girls)
|Karate at home|
Pros: unlocks karate themed items/wallpapers when levelling up
1. Yellow Belt
2. Red Belt
3. Blue belt
4. Purple Belt
5. Brown Belt
6. Black Belt
Main Hobbies Overview
I have closed the comments for this post, if you are having trouble, please use the unofficial forum, there are many people there who can and likely will answer your questions: http://simsfp.perturbee.net
Technical support (if your game does not work as expected, like no goals, no building options, crashes and such) use: https://firemonkeys.zendesk.com/