You can find several Pokémon from previous games in Pokémon Brilliant Diamond and Brilliant Pearl, giving you the ability to bring many Pokémon from previous generations. In this game, you have the chance to catch and find Chikorita, Cyndaquil and Totodile, Gold and Silver's starter Pokémon. Here's what you need to do to find Chikorita, Cyndaquil, and Totodile in Pokémon Brilliant Diamond and Brilliant Pearl.
The only time you will be able to encounter these three Pokémon is after beating the game. After that, you need to reach the end of the game and complete the entire Sinnoh PokéDex to unlock the national PokéDex. When you receive the National PokéDex, these Pokémon now become available to you and you will be able to find them.
Chikortia, Cyndaquil, and Totodile do not appear in the wild. Instead, you'll need to find them by exploring the Grand Underground. You can find them in their respective typical areas. For example, Totodile has the ability to appear in the Water areas, while Cyndaquil can appear in the Fire areas of the Grand Underground. You can increase the chances of these Pokémon appearing by placing Statues of the same Pokémon type in your secret base. You may need to explore these places a bit to encounter these Pokémon on your travels.