If you as a leader do a great job defining expectations, training new volunteers and enforcing the status quo, there's a good chance that you will never have to fire a volunteer.
If you are lacking in any of these areas, chances are it's your fault to begin with.
I witnessed a situation once where a new staffer was hired with no experience, no training, insulting salary and zero support. Soon, discussions of her falling short started echo through the meeting rooms. I must have sounded like a broken record, but I couldn't leave it alone. "What training have we provided her with? Has she been evaluated and critiqued? Has anyone tried to help her at all?"
To make a long story short, it all ended on a teary Sunday afternoon when she was fired in front of other staff. Totally caught off guard, this poor girl was shocked and devastated. "You just don't have what it takes to teach kids." This was the message given to this poor girl who's major was Elementary Ed by the church.
FAIL.
We as a church had failed to do our job, but instead blamed it all on the clay we were supposed to mold. 'You just don't have what it takes to become a pot." Said the potter to the clay.
Makes just as much sense eh?
All that being said, when you DO do your job and check all the boxes and you STILL have a lackluster volunteer - you owe it to the kids to fill that spot with someone passionate ans on-board with your ministry's vision.
Try this:
"[Volunteer's name], I love that you have the willingness to step in and spend time with our kids, but a willing heart is just the beginning of what we need to fulfill the mission that God has called us to. I need all of our volunteers to [fill in the blanks here with your expectations. For example: Show up ten minutes early, be familiarized with the curriculum, be prepared, organized, etc]. I've noticed that you have trouble coming through on some of these. When you first committed to this, you committed to [insert length of time]. Do you think that you can fulfill that commitment to the level I just explained? If not, its ok to let me know right now and we can work something out. I'd like to understand your thoughts on this. [Wait for response]."
If your volunteer can't fulfill the commitment, ask that they stick around until you have found a replacement. Give yourself 2-3 weeks max.
If they seem to have an awakening, tell them that you are elated and that they are on a 30-day trial period and that you will meet back with them after that time to re-evaluate.
More than ANYTHING ...
YOU MUST FOLLOW THROUGH.
Yes, this approach is assertive, but you are the LEADER. If you don't assert the needs of the ministry, who will?
Give it a try and let me know how this works for you!