Challenges for consent and community engagement in the conduct of cluster randomized trial among school children in low income settings: experiences from Kenya by Okello, G, Jones, C, Bonareri, M, Ndegwa, SN, McHaro, C, Kengo, J, Kinyua, K, Dubeck, M, Halliday, K, Jukes, M, Molyneux, C, Brooker, S