Children can learn from a very young age. It depends what type of formal practice you are talking about, and how long. Certainly we wouldn't want a three year old to have to sit for one hour, that would be too much. Sometimes if you teach them in a fun sort of way then they might want to sit with you for five minutes. You may want to talk about Lovingkindness, that is a good place to start. Learning how to think nice thoughts of their friends, of their family, etc. Often Yoga practice is something that children like and, perhaps that would be a way to introduce other wise thoughts as well.
Don't expect their attention span to be very long, you have to have short periods of it. Getting them to try to really taste something well, you can make it into a game. Sometimes you can introduce them to Compassion/Lovingkindness thoughts by taking them and observe the creatures, by trying to get them to imagine it all. So, it has to be on a practical level, introducing the Dhamma to children, rather than in a formal abstract way. They have good imaginations. This is why I say not to underestimate their ability. In fact, they often have a very good ability to be in the present moment, if it is encouraged and it makes things interesting to them. Everything is new to them, so they are less likely to be bored like adults.