I looked at Casa Dei Bambini Montessori For Children at Newton Road too. They are probably the most "montessori' in singapore. like a poster mentioned all of their staff is AMI which is the best certification there is for montessori training. they are also very organised in their mgmt. they know what they are doing and they have schools in hkg and canada. one reason i did not chose the school is that all of the teachers are of one nationality. not that i mind because they all speak perfect english, probably better than mine. i just wanted my son's school to represent the wonderful mixture of nationalities in singapore (chinese, malay, indian, european, etc) in terms of teachers and students. there are 2 more schools run by AMI principals but for various reasons i did not chose those school either.
again, you need to ask about the teachers qualifications. some centres only the principal has the qualification.
MMI is an organisation that provides certification AND schools. unfortunately i believe the schools are franchised and the one on claymore just became a montessori school. as such not all the teachers are qualified yet. for me, i felt the school was run more like a business; trying to maximising the number of students.
when researching for a montessori school, try to look for 3 hour programs (anything else may be too short). the option for an extended day program is also wonderful.
JMHO