Health officials say some of the children suffered fractured bones and "facial trauma", after being trampled on by the horses.
Two adults and 15 children have been injured by a stampede of horses at a riding school in northern France.
The accident occurred near the town of Vitré in Britanny on Tuesday evening, said local authorities.
Three children -- aged between 7 and 11 -- were hospitalised in a serious condition, but doctors now say their lives are not in danger. Two of which were transported to hospital with an air ambulance.
"The prognosis today is favourable," said Professor Louis Soulat, head of the emergency department at Rennes University Hospital on Wednesday.
The 17 people were evacuated to hospitals in Rennes, Fougères, Vitré, Cesson Sévigné, Saint-Grégoire and Pontchaillou for head and body injuries.
According to investigators, the children and two female activity leaders were taking some 20 horses to a field near the stable.
"On the way, one of the ponies -- probably the lead -- turned back for an unknown reason and was followed by others," Rennes public prosecutor said in a statement.
"In their flight, the animals pushed and knocked down the group," it added.
Some of the children suffered "facial trauma" after being hit by the horses' hooves, while others had fractured bones.
Two adults, who were there to supervise the children, were also suffering "post-traumatic stress", Soulat told reporters on Wednesday.
Firefighters were called in around 8:50 p.m. local time to the site near Vitre, which Le Journal de Vitre reported was hosting a summer camp of 26 children aged between 9 and 13 from all over France.
An investigation into the incident has been launched.