I would steer clear of Dexters if you want docile and able to milk. We had 2 Dexter steers last year and never managed to tame them enough to handle, in spite of daily bucket feeding.They would also jump out of their field, spooked easily, and broke fences. Originally I wanted a Dexter cow in-calf or with calf at foot, as a house cow, plus a steer to raise for beef, but couldn't find one for sale. Didn't manage to find any Dexters that were being milked within a 50-mile radius - round here they seem to be kept just for beef. I also considered Jerseys, but couldn't find any for sale within a reasonable distance and at a price I could afford.
I now have an in-calf Shetland cow plus steer, and they couldn't be more different - docile, friendly, and welcome human attention. She's not been hand-milked before but I handle her daily and she is happy to be brushed and stroked all over, including her udder, so am hopeful!
It's fine to choose your ideal breed however you also have to consider what is available and the cost - personally I wouldn't want to go more than 50 or 60 miles away to buy cattle, both for my sake and for the sake of the animals travelling.
Also, regarding children - I wouldn't trust ANY of my livestock around unsupervised children, however docile they are. My lovely cow caught my hip with her horn the other day, when she swung her head round to look at something and it's still sore. My fault for standing too close and not being sufficiently observant. None of my animals is aggressive however they are not aware of their own strength and could easily injure a child inadvertently.