Still, federal and state officials and health providers say that vaccinating children is likely to be a more challenging process than it was for adults and teens. The federal government plans to allocate shots in the initial rollout according to a formula to ensure equitable distribution, likely based on a state’s population of eligible children, according to a federal health official who spoke on the condition of anonymity to share planning. Enlisting besieged health providers and persuading reluctant parents will complicate the process.