It is unlikely an allergic reaction
to Milk of magnesia
. If it is so it would not have been limited to tonsils alone but it would have appeared on lips and/or mouth. Secondly lymph glands would generally not get swollen.
It is most probably due to infection either viral or bacterial.
Whether an associated sore throat
, difficulty or pain on swallowing, fever are there?
You have written it is 4th day and did not mention of other throat symptoms or treatment given for that. So I assume that is not severe or is subsiding. In that case it could be viral in origin. Then probably nothing else need be done.
If neck glands are quite big a viral infection called Infectious Mononucleosis
need to be ruled out
In case if she has above mentioned symptoms you need to do the following:
A swab can be taken from patch of tonsil for culture and sensitivity test ( by a doctor)
Plenty of warm oral fluids (water milk etc)
if fever is there
Antibiotic may be needed if symptoms are increasing. In that case better show to an ENT specialist