You mention "they have the same brushing habits." Are you implying that they are responsible for brushing their teeth on their own?
Not meaning to preach, but if the dentist hasn't already said this, you should still be the one brushing their teeth for them. Children do not have the dexterity or ability to properly brush their own teeth until they're about 4 or 5, and still, most dentists would encourage you to follow-up after to be sure their teeth are properly cleaned and flossed.
Our dentist is adamant that parents brush their children's teeth until they're 6! He's even told us he has no problem holding his children tight to get the job done. Thankfully, as much as my kids don't like brushing, I've never had to go that far, but I have had an easier time getting them to brush with an electric brush and kid friendly toothpaste and washes.
If you aren't brushing their teeth 3 times a day and are just leaving it to them to avoid a fight, this may be the reason your son has tartar build up on his bottom front teeth. Calcium build up is another way of saying plaque build up. That happens when teeth aren't properly brushed and is the stage before decay. While these are just baby teeth, the damage of tooth decay is far reaching and can effect the health of his adult teeth if he gets cavities now.
To be sure you aren't leaving anything behind when brushing, there are rinses (even for kids) that temporarily stain the teeth so you can see where plaque has formed prior to brushing. The stain comes off along with the tartar as you brush. You can find this where mouth wash and toothpaste is sold for children.
Be sure to floss too. There are flossers made for kids also found in the same area as the mouthwash. Flossing also keeps the tartar away.