Below are a list of useful websites to help junior chess players improve.
Beginner Sites
www.kidchess.com/ - An American site which teaches the basics. Good for practising simple checkmates.
https://lichess.org/learn#/ - Learn to play chess with Lichess. No need to create an account but if you do it is strongly advisable children under 13 enable kid mode.
https://chessplus.net/interactive-games/ Play mini games without all the pieces.
Beginner to Advanced
Chess-endgame-trainer - Practice your endgames from 2 Rooks vs a King up to advanced endgames.
https://www.chessinschools.co.uk/chess-at-home - Worksheets, videos and quizzes by Chessinschools a UK Charity.
https://www.chesskid.com/ - Play and learn chess online. A safe site for children but you need to pay for premium features.
https://www.delanceyukschoolschesschallenge.com/ Find out about competitions and coaching.