Your small folk need a playground. In Aix-en-Provence, there are three main options. The most lovely, historic and central is Le Pavillon de Vendôme in the Aix old town. There’s a gorgeous 17th century mansion. Don’t go in. It costs money, it’s not the big draw here, and the staff is grumpy.
Do enjoy the (free) formal French garden with fish pond, flowers, plane trees, and petit playground with picnic tables. This is where many an Aixois family comes to catch up with friends, or Aixois grandparents babysit little ones while the parents go for apertifs in the old town.
Other options: La Torse for a large, wild park with a stream and jogging paths a good hike from central Aix. Parc Jourdan, 15 minutes from central Aix north of the Mazarin quarter, for the most extensive playground and an international set of parents and nannies.