I served as 1st Councilor in Primary for 4 years. We never ran out of Volunteers, no matter what the age.
Make up a chart with ALL of the childrens names on it and columns for each Sunday for the year. Mark out the April and Oct. conferences, then your ward and stake conferences. Put stars or stickers next to the sunday the child gives a talk. You could also do columns for prayers and scriptures.

When you ask for volunteers for the following month ~ be sure to check the chart to see who has just done it, who hasn't yet. Don't tease, cajole, or embarass those who haven't done it. Just ask if the Holy Spirit has prompted them yet to give a talk/ prayer/ scripture?

Do you give them topics?? As an Adult giving talks, I much prefer that the topic be assigned. You could have the Children suggest topics - then remember who suggested which topic. Then when she/he hasn't volunteered yet you could say: Patrick, you suggested that there be a talk on Where to pray - the Holy Ghost must have prompted you to say that - do you think so? Do you think you are prepared to give a talk next month on Where to Pray?

Take the suggestions the children give and match them up to a corresponding monthly theme. Give them at least a month to prepare their talks, and call them at home with offers to help them, and follow through with them no matter what age they are. Some of THE most spirit filled talks were from our three year olds!

Good Luck