The short answer
A courtyard riad with a pool in the medina, or a resort hotel in the Palmeraie. A riad gives you a calm, enclosed base near the sights; check the pool depth and the stairs. A Palmeraie or Hivernage hotel gives lifts, bigger gated pools and family rooms, with cars at the door. Many families split the stay.
In more detail
A riad is the classic choice and works well with children. The enclosed courtyard gives them room to roam while you rest, there is no busy lobby, and staff tend to be delighted by children. Choose one with a plunge pool or garden courtyard, and check the depth of the pool and the steepness of the stairs before you book. A ground-floor room removes the stair worry.
A hotel is easier for some families. Hotels in Gueliz, Hivernage and the Palmeraie offer more space, bigger pools, lifts and kids' facilities, and taxis and cars pull up at the entrance, which matters with luggage and pushchairs. The Palmeraie resorts trade medina atmosphere for gated shallow-end pools and ground-floor family rooms.
Location within the medina matters too. The quieter northern medina around Mouassine and Dar el Bacha, or Bab Doukkala, is calmer than the souk core but still walkable to Jemaa el-Fnaa.
Splitting the stay is a popular compromise: a couple of nights in a medina riad for the character, then a few nights somewhere with a bigger pool. Tell us the ages and we will suggest places.