The amount of sleep required for children in the Learn to Train stage is generally somewhere in the range of 9.5 to 10 hours per night plus a 30 minute nap between 2 and 4 pm. In addition to requiring this amount of sleep it is also important that they have a regular sleep routine / schedule. Sleep should be in a comfortable sleep environmental and should be precipitated by a regular 15-30 minute bedtime routine.
The following links point to resources that have been collected and are relevant to this skill (?).
Sleep, Recovery, and Human Performance, is a 20 page pdf document put together by researchers in Calgary for Sport for Life. It discusses the importance of sleep and recovery for sport performance and some of the most prominent sleep disorders and factors that affect one's sleep and recovery. The document is very clearly and consisely written to discuss the key factors across all LTAD stages.