What is the medication of choice to prevent Lyme disease in a child less than 8 y/o who has been bitten by a tick?

Nothing.  In children less than 8 y/o and pregnant/lactating women, where doxycycline is contraindicated, NO prophylactic antibiotic is recommended.  There are no data to support the efficacy of another antibiotic other than doxycycline. 


Source

Sexton, D.  "Evaluation of a tick bite for possible Lyme disease"  Up to Date.  2011 Jan.