For parents can't speak French, send grade one kids to French school is not at good idea at my opinion. Because kids have to learn everything in French and you can't help them. It is important for kid do well in grade one, so they would build confident for themselves. Good starting point is middle French immersion at grade 4. At that point, they already do English well and for good student they already have a good learning habit. You can let them try one term, put some money for private French tutor, if they can’t keep up, pull back to English programme. In middle French, there is still English class and math is in English, so kid would keep up with their English and math.
Since parents usually only send good student to MFI and would pull them back if they can’t take it. The kids stay there usually are the best from English programme.