What will you do if your child wants to play with older kids but she still can't follow the game rules, thus the older kids are reluctant to include her.

How about you set up your your younger child in his own corner to play the same activity away from the older kids? For instance, if they are colouring give him his own spot, crayons, and paper. If they are playing a board game give him his own corner and his own game. Perhaps playing simultaneously nearby will make him feel included.