First thing is, you need to find out exactly what happened.
Contact the school on Monday and arrange to meet with the Headteacher.
They will need time to find out what happened, as it sounds like there were several possible failures (how the injury occurred, the decision to seek or not seek treatment, informing parents, possibly others you are not aware of) and the school needs to find out where these occurred.
There should have been a system in place whereby a senior member of staff was available in the UK for the staff on the trip to contact in the event of an incident such as this occurring. The decisions taken should have been jointly made, given the implications if the child was deemed not fit to fly (staff to remain, additional accommodation, re-arranging flights, contacting parents), not left to the trip leaders alone who were already running the trip and dealing with the incident.
The Headteacher will need to find out if this system was in place and whether it was used. It's possible that some of the failures were at this point, or at the planning stage, e.g. were there enough staff, leaving the staff on the trip with few options other than to do as they did.
You should expect the Headteacher to inform you of when they will get back to you with their findings, which may be a few days.
If you are not happy after the Headteacher 's investigation, then you need a copy of the school's Complaints Procedures, which will tell you what to do next.