Sleep-away camps are not covered and you can't prorate the time, either.
Camp, Overnight
The cost of overnight camp is not reimbursable, even when the daytime portion is broken out. Overnight camp is not considered an eligible expense by the Internal Revenue Service (IRS).
http://www.aetna.com/members/fsa/eligibleExpenses/depcareFSA/dependentcareFSA_C.html