The group was attending a youth team's football trials at a recreation ground when they were caught in a violent storm.
Fifteen of the injured were taken to Maidstone Hospital in a fleet of ambulances. The Kent air ambulance made three trips.
The seriously injured boy, who had not been identified at press time, was being treated at Billericay Hospital, Essex.
An ambulance service spokesman said at least three of the victims had received very serious burns. One male patient had been left with severe facial injuries and up to 40 per cent burns.
"Thanks to the air ambulance we managed to get the most seriously injured to hospital very quickly," said the spokesman.
Kent fire brigade answered several reports of lightning strikes across the county yesterday. Houses were struck in Ramsgate and Folkestone and Pembury Hospital, near Tunbridge Wells, was also hit. But in these cases no damage or injury was caused.Reuse content