All members of your party, including small children, are included when Disney determines your head count for a meal. For head count purposes, your group consists of six people.
Eighteen percent is now the national standard rate for tipping at table service restaurants. You can read more about this
HERE.
The good news for you is that while your group will be assessed the standard 18% gratuity, because your two-year-old will likely not be ordering much (if any) food, your food cost base will be fairly low. Let me explain --
For parties of six or more on the Disney Dining Plan, the 18% rate will be assessed based on WHAT YOUR MEAL WOULD HAVE COST IF YOU WERE PAYING IN CASH. Since your little one's food will not add much to this total, you will be paying less of tip than you would have if you were dining in a party of, say, six adults.