It can be difficult for children when they are separated from their friends and feel like the odd one out. You can help your son by encouraging him to make new friends with the children in his class, but also making an effort to create other opportunities for your son to play with his friends in the other classroom outside of school time.
Your daughter's willingness to open up to you about her friendship woes says very good things about your relationship with her. Above all else, keep those lines of communication open so she will continue to share with you.