Hi Katherine,
From the way you're asking this question, I'm guessing that you mean
Tomorrowland Speedway, located in the Magic Kingdom, not Test Track located in Epcot, since you are interested in taking your little one with you to It's A Small World. You will probably have to ride with your 3 and 4 year old children, as the minimum height for children riding alone on this attraction is 54 inches. The minimum height for riders for this attraction is 32 inches.
In the instance that you did indeed mean
Test Track, the minimum height for this attraction is 40 inches and children younger than age 7 must be accompanied by a person age 14 years or older.
You can utilize the
Rider Switch service for both attractions I have mentioned above. You and another adult can trade off with your little one so you can each experience the attraction with your eligible child or children.
Please come visit us again should you need any more advice on your upcoming vacation!
Best Wishes,
Amy B.