It is difficult to recommend an assessment tool because the placing of students in different levels depends very much on what your syllabus is like. Wherever I have worked, I have used an assessment tool produced by the teachers of that school. In this way, it is easier to link the students’ ability to what is taught in the classes.
I suggest you read articles on writing tests and try to produce your own based on how you are going to teach. For example, if your classes will be mostly oral, then you will want to place students according to their spoken English. Whereas if your classes are grammar focussed, you will want an assessment tool that covers grammar more thoroughly.
Finally, for children I think it is best to group them by age as much as possible. Children mature quite quickly especially regarding motor skills such as colouring in and cutting out. If students are grouped too rigidly according to level, you might find you have a ten-year old in the same class as a six-year old and this doesn’t work. Children of those ages have very different interests and abilities. It is fair easier to adapt material for a group of children of the same age and mixed ability than it is to work with mixed ages and the same level. So, I suggest you also bear in mind the age of the children as well as the level.