just wanted to chime in and say that it is fairly common and age appropriate behavior for a 3-3 1/2 yo to want to be a baby again, regardless of if there is a new baby involved. My 3yo did this (with a new baby involoved, lol) and I was a bit freaked out and thought it was all about the new baby, but then learned that others I knew who had one kid the same age were doing it too, so it wasn't my newborn bringing it out in her, just a normal devleopment. Once I started thinking about it more, it made sense. What a huge leap in development, all of a sudden they CAN do things themselves, but will Mama still want to help them? or will she help them if they do need it at some point? or if I don't need Mom for this, does she still need me? I think it makes a ton of sense.
Also, my now 4yo can get ready to go outside by herself at pre-school (snowpant, coat, mittens, hat, gator, boots, the whole Minnesota winter thing), but at home still insists she needs help. Different expectations at school, plus at home, I only have to help two kids get ready for the outside, I have the time (even if it doesn't feel like it), not 18 3-5 yos running around like maniacs.
Not that this specifically addresses your montessori question, but I felt like responding to the other thing. Sorry if it's too off topic. Let's see, how to bring it around back on topic...my dd will most likely be starting a Monessori program in the fall, there!