There are many excellent lessons available on chess.com.
Working through ALL of the lessons on chess.com in order will give you a solid chess knowledge. But this would take some time.
Here are some direct links for the most relevent lessons for juniors on chess.com
Theme Description
Section with lesson on each chess piece Group of lessons on each chess piece plus setting up and pawn promotion
Check, checkmate, stalemate etc Group of lessons covering check, checkmate, stalemate,
Piece value Piece value ... we use this to help decide whether to trade pieces
Castling Castling
En passent En passent - if a pawn moves 2 squares to try to jump past an opposing pawn on its first move, the opposing pawn can take it as though it had only moved one square
Theme Description
Forks A fork is when we attack 2 valuable pieces at the same time i.e. Knight attacks King and Queen
Pins A pin is when a piece cannot or should not move because a more valuable piece is behind it. i.e. Queen in front of King
Skewers A skewer is when a piece has a more valuable piece in front of it and has to move i.e. King in front of Queen
Bad trades Showing how we use piece value to decide whether to trade pieces
Remove defender Showing how we can win a piece by removing a defender
Trapped piece Showing how we can sometimes trap a piece such as pushing up pawns
Discovered attack Discovered or revealed attack when moving a piece reveals an attack on another piece
Discovered check A powerful form of discovered attack
Powerful moves Combining tactics
Combinations Combining tactics
Section on trades Capturing pieces
Section with more tactics More tactics Sac, Double check, Smothered Mate
Theme Description
Scholars mate 4 move checkmate
Winning with Two Rooks Rolling rooks ... push the King to one side
Winning with Queen and King Queen boxes the enemy King and reduces the box
Winning with Rook and King Rook boxes the enemy King, supported by our King, and reduces the box
Checkmate in 3 moves Examples of FORCED 3 move checkmates
Other checkmates Pawn race, Back rank, f2/f7, King safety, Dont just check
Theme Description
Opponents move Opening principles
Pieces working together Making the most of your pieces
Choosing the best move Section containing difficult positions, good and bad pieces
Use pieces effectively Section containing lessons on each piece
Theme Description
Control the centre Opening principles
Your opponents move Opening principles
Develop your pieces Opening principles
Opening principles section Active pieces, opponents move