Both foam and spring mattresses are suitable for toddlers. Foam mattresses tend to be lighter and easier to transport, which makes for easier shipping, delivery, and maneuvering within the home. Coil or innerspring mattresses tend to be more durable.
But if you’re looking for the best of both options, a hybrid mattress is the way to go. These offer the durability and support of spring mattresses, but as they’re topped with a foam layer, they are a little lighter and may be more comfortable for your child.
If your toddler tends to be a hot sleeper, innerspring mattresses allow for more airflow and a cooler sleeping surface.
A newer alternative to spring or foam mattresses is latex mattresses. All-natural latex mattresses are more eco-friendly than other options while providing a more supportive rest (source).