Children can learn about and accept the different "roles" of mom at home and at work/church group/sports team/....
There are scores of dads coaching their children's teams, there are parents (or even siblings - much more potential for conflict and less emotional maturity involved!) leading church groups, boy scouts, dance classes, orchestras... There are pastors whose children attend sunday service and pediatricians who treat their children. Kindergarden teachers having their child in class is not different.
Admittedly, there are instances where (often) the parents can't handle this situation professionally, but as your wife should be able to avoid favouritism in her daily work anyway, I see no problem.
Your child knows how a kindergarden works, so just explain that mom is the kindergarden teacher in the morning (or whatever time frame applies), and that she is there for all children in the group. And of course it's your child's "job" to be a kindergardener in the morning, too. After kindergarden, they go home and are both finished with "work" and are Mom and child again.
Besides, this will be a temporary solution, so I wouldn't worry too much, even if some things don't go smoothly.
Just for reassurance my personal experience:
The son of my kindergarden teacher was in my group too - never a problem. Yes, he did call her "Mom" usually, but otherwise no problem, ever. (Small kindergarden with only one group, so this was the only feasible solution.)
