There are a couple of options, as mentioned the Honda Youth Challenge is an option althougth realistically it is the practice ahead of one of these that develops skills rather than the competition itself. We had intended putting a schools entry in this year but have struggled with time so will need to wait til next year. There is the K-Class kids powerboat racing that takes from aged 8 upwards and there is training to be able to enter that - i believe you don't always need your own craft for this either nowadays.
Another worthy option is the Sea Cadets or Sea Scouts. We do alot here with Parkstone Sea Cadets and they have a great time afloat across a variety of craft and learn some really good skills.
Your best option i would say though is letting her helm your craft alongside you with her taking increased responsibility for it. Has she spent enough time at the helm to do all the things she learnt at level 1 with ease, can she always perfectly come alongside a (dummy) mob, can she pick up mooring buoys, can she berth the boat in a marina in all manner of berths and conditions etc etc.
I would also suggest getting her a good book on the subject and teaching her a bit yourself, give her her own charts and just chat through the features on them with her and you'll inspire her to spend time studying them. You might also think about additional Level 1 courses in different locations on different craft. Teach her lat & long & start to set her plotting challenges etc etc
We have a similar challenge with our 9 year old, she's done Level 1 three times, has been a guinea pig on numerous instructor courses but ultimately what she gets most from is simply the practice aboard the craft. As some will attest to on here a 9 year old perfectly berthing a 8.5m scorpion in a marina is a cool (at least from a fathers perspective!) skill for your daughter to have!
Do shout with any further questions