The National Network for Children (Bulgaria) shared advice from UNICEF on how parents can maintain the mental health of children who go to school during the new COVID-19 measures implementation.
It's advised to calm down the child by talking openly to him/her about his/her worries and that feeling anxious is natural. Tell the child that you know about his or her concerns about Coronavirus and that talking about worries and feelings is healthy. It is recommended that you discuss changes at school with the child, such as the need to use a protective mask. It can also be a challenge for children to keep their physical distance from friends and teachers at school. In this case, you may have to encourage him/her to consider other ways to communicate safely with his/her peers. Children need to be reassured about safety measures that help students and teachers stay healthy. Remind them that they have just as important of a role in curbing the spread of the virus by washing their hands with soap and coughing or sneezing in a bent elbow as much as the adults. Remind them of the positives - that they can meet friends and teachers in the classroom again and that they continue to learn new things.
You can find the detailed text with the advice on the topic on the NNC Bulgaria's page (the source article is in Bulgarian).