So, get a letter with scan appointment saying don't bring other kids as they will have to wait outside with your companion and definitely cannot come into the scan room.
I totally understand and support this as the scans are not for fun but for medical reasons.
At both my 12 and 20wk scans people have turned up with all manner of kids in tow (with partners etc who could have waited outside so its not that they didn't have a choice but to take the kids in) and have gone into the scans with the children of an age that might well be disruptive.
Why don't they enforce the rules? Seems a bit unfair to those who make the effort to find childcare....