If we could compare a child to something, it would be a sponge. Children at a young age absorb everything, the more they do the activity the quicker they are able to do what it is they are learning about.
By teaching children a second language at a young age you are allowing their brains to expand and allowing them to have an extra foot in the door for when they are older. Children thrive on new activities and by allowing them to explore and gain new experiences they will be better equipped for the future, whether it is in the work force or travel having a second language such as English will really help them in their endeavours.
With learning a foreign language they will also be able to connect with other people from other countries, broadening their horizons.