I was a MSA (Midday Supervisory Assistant) until very recently. We also had 2 MSAs to 90 children, but this is NOT the correct ratio and NEEDS to be brought up with the Head or Governors a.s.a.p.
If a child is injured, the MSA should react as calmly as possible and should also be First Aid trained. It only takes one MSA to deal with a child with a bleeding face, not two.
Mobiles should not be used whilst on duty except for emergency situations.
Sitting down on a bench is unacceptable - the MSAs should be up and initiating games with the children, observing play and listening to children.
Yes, MSAs are poorly paid and in my experience receive no support whatsoever from other staff, but that is not an excuse to do the job badly. If an MSA isn't happy to get in there and play with these little ones, they simply shouldn't be MSAs. I would definitely put in a formal complaint in this case.
Perhaps it could be suggested also that the MSAs do the course for MSAs which lasts a few weeks and is very, very thorough, and they get paid very well for doing it. We had to do it, and it was well worth it. We all became much better at our jobs after doing it. It was an accredited course run by the WEA - webcam.wea.ac.uk/coursesearch.asp
Hope this helps.