Not at all, says Diane Morton, director of the School for Young Children at Saint Joseph College in West Hartford, CT. In fact, these kinds of grammar goofs can be a good sign. "They show that your preschooler has picked up on word rules, like adding an 's' to pluralize a word and adding 'ed' to make a word past tense," says Morton. But your kid doesn't know all the exceptions to those rules yet. So since he has hands, arms, and legs, he may think he has "foots," not feet.
By age 4, your child will have a better handle on which words have special rules. For now, help him by responding to him with the right word when he uses the wrong one. For instance, if he pushes his plate away and says, "I eated all my lunch," you can respond with "Wow, you ate all your lunch. I ate all my lunch, too!" No matter how tempting it is, try not to reinforce his adorable mistakes by repeating them back to him.