The Children's or First Catechism (it's been published under both names) is an excellent little catechism aimed at young children. Highly recommended for that age group.
I could say more if I knew what you mean when you say your church is transitioning from one to the other. Those two catechisms are not really replaceable with each other. The Westminster Shorter was created to be manageable for kids, but it is robust enough to be used by adults as well. The Children's or First Catechism is much, much simpler and is clearly for children. Although I agree with Grant that adults can benefit from its simplicity and directness, I would hope that adults and older kids will also want more depth and expanded teaching.
I suspect a decision to replace the Shorter with the Children's would not have much to do with doctrine but rather with age-appropriateness, or learning levels, or the willingness of the users to commit significant time and effort. Maybe a church believes its kids would find the Children's more manageable or more understandable, so it decides to start using it in children's classes or to start recommending it to families rather than the Shorter. The wisdom of this would depend on the age of the kids involved and the ability/willingness of parents or teachers to work with them.
So the situation you have in mind matters. It would be a cause for concern if this transition is happening in the officer's training class, but a sign of wisdom if it is happening in the preschool classes. If the situation is something in between, there are probably pluses and minuses for the church leadership to consider.
