Whether you use the lectionary for children's sermons or not...
In my opinion, the Children's Sermon in worship is something that so many churches mis-use, -turning it into an opportunity to speak to the adults, and using the kids as props.
I think this happens more often when the pastor didn't prepare.
If it really is FOR the children, then the selection of scripture needs to ALSO be FOR THEM. This means that many lectionary texts simply are not appropriate.
When we do the Children's Sermon here, it usually connects to something happening in worship or a specific theme of the church season. ie... very tangible for the kids.
Jan S