When you play Daily Adventure or Mission, you can collect monsters. You can see the monsters you've collected in Monster Collection by tapping the monster button at the bottom right of the menu screen. If you can't see the monsters, try the following two things.
Make sure you are signed in to the correct account.
Monsters are stored separately for each user. Make sure you are signed in with the same email account you used to play. If it's the same, make sure that you have selected the user you played as in the past. You can select a user by tapping the user name on the start screen.
Make sure that you are on the same level as when you caught the monsters.
Monsters can be captured at levels A, B, C, and D. Has your child leveled up to levels E, F, or above? On levels E and F, the monster menu does not appear on the menu screen. Even in levels A, B, C, and D, the monsters are stored in the monster menu of each level, so make sure you have selected the same level as the level you played before. If you get all monsters of that level in Daily Adventure, you will automatically level up to level D. If you level up, you may not be able to see the monsters from the levels you played before. If you select your previous learning level, you can see the monsters you have collected again.