The schools are very strict on this now . Birthdate cutoff for some school divisions is September 1, not year end as in Manitoba, so some Manitoba kids will be younger than the US kids. School districts vary greatly in the type of programs they offer, the size and structure of schools, teaching models, etc. Check around for what they all have to offer. Again, Schoolhouse Magazine lists all this information.
School starting times vary greatly.
This may affect your work schedule, and is worth looking into. Any time you cross the border, take the kids' entry Visa form along, as well as birth certificates. The border folks may not let you back into the US if you don't have entry Visas for everyone returning to live there. If you are a two parent family, it may not be a good idea to put your kids on either of your Passports. The Canadian passport office mentioned that this can cause problems if the parent without the kids listed on the passport goes traveling with them.