My 7 year old daughter is really interested in money, so I introduced decimals by introducing money, and then playing with "buying" and "making change" in her pretend store. She doesn't know she is working with decimals but she's able to make change really accurately. I actually have no idea if this is the "right" way to work with decimals or not...
I introduce the decimal system through powers, and objects that model powers, such as fractals. One of the best models for going in and out through the powers is ZOOM. Children these days experience a lot of situations with zoom in virtual environments, cartoons, telescopes, etc.
This Powers of 10 in the Universe video is a classic:
In a Montessori classroom math is part of the whole. Developing a "mathematical mind" is not done separately from the other essential aspects of the curriculum. There is much that comes before the introduction of the decimal system and much to follow. To pull one lesson from the entire experience does not allow for a fair consideration of the whole; however there is a lot to be found online. Here are two:
Montessori Math Lesson - Introduction To Decimal System
Everyone Should Learn Math the Montessori Way