How small is small? If they’re old enough for booster seats, you can get inflatable boosters, and grab a regular Uber or Lyft. If you need car seats (and don’t have them), you might consider booking a car fro Disney to Universal. The same companies/drivers who take families from MCO to the parks can take you between the two parks. Of course this assumes you don’t have a car because driving yourself is the most straightforward option.