In some cases crowns are recommended. There are several reasons we may suggest a crown; most common is the size of the decay and consequent lack of tooth structure. Composite restorations have to have adequate support from natural tooth structure. For example, a ratio of 50:50 restoration to tooth structure is unfavorable for long term retention and function. If a cavity takes up roughly half of the tooth surface a crown is required.
The next most common reason we may suggest a crown is the location of the cavity within the tooth. The pulp, or nerve, extends to certain areas within each tooth. Decay that cannot encroach on this part of the tooth. If it does 
the nerve has to be treated and a crown is what will allow the tooth to remain in place until it is naturally lost.
One common question we hear is, "Why not remove the tooth?" Removing teeth early is rarely a good option for it reduces a childs ability to speak, eat, and develop craniofacially. Baby teeth are important to facial development as well as permanent tooth development.