My son is 11, homeschooled, and serves Mass on Wednesdays when we get together with our homeschool group. The priests at that Mass do not allow females on the altar as servers. Then we go to church on Sunday at a parish close to our home and the priest allows women on the altar. This lack of consistency is confusing my son. He knows that in Rome women are not on the altar with the Pope. I do not want him to think bad of Father for allowing women to serve at the altar. What can I tell him?
Explain to your son that the Pope has allowed bishops to permit female altar servers. If an individual bishop permits female altar servers in his diocese, a priest can permit them to serve at the Mass. Apparently this is the case in your diocese: With permission from the bishop, some priests in the diocese allow female altar servers and others do not. It is an issue on which the Pope has allowed local dioceses the freedom to choose to allow female altar servers.
To make this more understandable to your son, you might make this analogy: As his parent, you can decide to allow him and his siblings the freedom to make decisions within certain specified limits. So long as they remain within those limits, the children may make different decisions according to their individual preference. In the same way, on this issue the Pope has allowed local bishops the freedom to adopt different policies. If a bishop decides to allow female altar servers, he in turn allows his priests the freedom to allow female altar servers. So long as the bishops and priests remain within the permitted limits, they have the freedom to make decisions according to individual preference.