I found this great advice on A Cup Of Jo:
I was chatting with a friend one day when her 3-year-old son wanted to say something. Instead of interrupting though, he simply placed his hand on her wrist and waited. My friend placed her hand over his to acknowledge him and we continued chatting.
After she had finished what she was saying, she turned to him. I was in awe! So simple. So gentle. So respectful of both the child and the adult. Her son only needed to wait a few seconds for my friend to finish her sentence. Then she gave him her complete attention.