My kids that are in M school are DD1 who is 3 (will be 4 in Nov.) and DD2 who is 2 (will be 3 in Oct.).
We only do half days because after lunch at 12:00 they take a 2 to 2.5 hour nap and then they have another work cycle or outside play (maybe both?) and then go home. This is also when they have more laid back time (birthday celebrations, yoga, dance, etc.) To me, it wasn't worth the extra $ to keep my kids there for a nap since they can do that at home. Plus, they usually like to spend an additional hour at home working from our own Montessori job shelf. However, our school is awesome and invited my almost 4 year old to stay until 3:30 every Wed. (she naps there) so that she could attend the dance class.
She *LOVES* it and talks about Wednesday every day!
When we first toured the school for DD1, they only had a 1 half day per week opening, so we snagged it to get our foot in the door. It was an extremely rough two months for DD because 1 half day per week was not enough consistency. It was like the first day of school EVERY Friday. She would cry every week and say she didn't want to go.
Then we got 2 days (lasted a month) then in the summer a lot of kids are taken out of school for those 3 summer months, so we got our 5 half days. I originally didn't want more than 3, but the lack of consistency was really confusing for her. It's so much easier for her now because she knows what to expect. And let me say that I was EXTREMELY against her going 5 half days. Extremely!! I'm a SAHM and she is my first born, so it was really more about me.
Now that she has been in school since Feb., I can honestly say that 5 half days is really better for the child. In fact, she misses out on so much fun and learning in the afternoon, but I'm still not willing to be away from her that long. Mommy time in the afternoon will always trump school.
DD2 only goes 3 half days and we are seriously thinking about 5 half days, but I think we'll wait until she gets moved to the 3-6 classroom for that.
I think your 5 year old will do great in the 5 half day program and maybe you can ask if they will allow 3 or 4 days for your 2 year old. Sometimes schools will bend the rules. One of the schools that was on our list has 4 and 5 day programs, but they said they would work with me and allow a 3 day program. Doesn't hurt to ask! Maybe explain when he is closer to 3 that you will want the 5 day program. Good luck!