Pop÷ Solo is 500 np's which is the only bubble popping game available in the DSiware shop that I'm aware of. This one has a lot of game modes but not much variety in the gameplay. I wouldn't recommend this one as none of the little ones never give that one a chance when I download it.
As for good 200 point games for a six year old its hard to say. I have a 8 niece who really loved games like Castle of Magic(500 np's), Oregon Trail(800 np's) and Mario vs. Donkey: MMA(800 np's).
I hear good things about these 200 point titles though....
1. Aura- Aura Climber
2. Spotto
3.My Farm
4. My Exotic Farm
As for a Electroplankton titles the most popular would be Hanenbow and Luminarrow. I've never played any of them though.
I'd go with Mario vs. Donkey Kong:MMA and Aura- Aura Climb if I wanted to spend 1000 points in the shop.
