Straws take a little while to get use to. And even when your kid knows how to use one they can still be a little problematic. Often a child will tip the cup to get there mouth around the end of the straw. And down comes the soda when this happens.
To head off this potential mess try clipping the straw shorter. Cut about a quarter of the straw's length off. If the cup is short (like most child sized cups at restaurants) you may need to take a little more off.
Carry a key chain knife with scissors and you are just a quick clip away from a mess free dinner out. Okay that might be a slight exaggeration. It will at least be a drier dinner out.