In the permanent dentition, an ideal occlusion consists of well aligned teeth in each jaw and a balanced well intermeshed bite of the cheek teeth as shown below.
In the mixed dentition, a good occlusion can have a variety of appearances since baby teeth are continually falling out and permanent teeth are in various stages of eruption.
However, there should be plenty of space for the second teeth to fit in and the cheek teeth should be in good interdigitation:
In a good primary dentition we would expect to see overlapping of the top teeth to the bottom teeth similar to the permanent dentition. Some moderate spacing between the baby teeth (called “primate spacing”) is also desirable. Children who do not have any spacing between their baby teeth tend to have a significantly higher chance of crowding in the permanent dentition.