Swallow attacks!

We live in mid-Cornwall and the swallows arrived about a week ago.  During the last two days the swallows have started to divebomb us on our field making very loud chirping noises!  It is actually very intimidating.  I am fearful for my two small children and do not wish to endure this for the whole summer.  Last year the swallows only showed this behaviour when they had chicks in their nest.  I cannot believe that they are nesting already after only one week!  Any suggestions?