Major difference is that the Declaration was a statement of ideals. According to Pearson it was necessary to transfer that statement of ideals to a Convention in order to be able to turn those ideals into laws.
The Convention defines the age of children as those under 18 years. It specifically addresses issues such as child labour, protection from violence and exploitation, the rights to have their own views and be heard, and to be treated fairly if accused of a crime. The Convention identifies the child as more of a participant than does the Declaration
There are differences that result from the different eras that the two documents were written, in terms of terminology such as handicapped and the use of the of he and him, exclusively.