Cinque Terre is a better known beach town but is not ideal for toddlers as it has cliffs and a lot of steps. Pienza is in the Val D’Orcia (a UNESCO World Heritage Site) and is a stunning Renaissance town. The views of the countryside from here are spectacular.
The hill towns of Chianti are also worth exploring. Greve in Chianti is very picturesque. Most of these towns are comprised of a center without cars, making exploration toddler-friendly. Bear in mind that many of the towns in Tuscany will have hills, steps and cobblestone streets so you may find yourself carrying your stroller at times.
If you are looking for a good beach holiday, Forte dei Marmi is an upscale, beach town on the border between Tuscany and Liguria and has sandy beaches, perfect for toddlers to paddle. The island of Elba, off the Tuscan coast is another great option for under 4s and offers stunning beaches and family friendly accommodation. Ferries to Elba depart from Piombino.