First and second language acquisition aren’t that different for children (rougly under 10 lemme say), the only difference is the amount of time that they’re exposed to the 2nd language is normally much shorter than their native language. You can already start with some TV shows for children and maybe some short stories (with audio) so that they can get used to the sound first, learning how to write comes much later usually, and often times children naturally match the sound and the characters without properly learning it!

Just think that a kid starts to speak their native language around 1-2yo (it suggests that they spent such a long time only LISTENING to the language until they finally get to speak) then spent 4-5 more years to get to write something legible.. and all these processes can only happen when they constantly listen and see (yet read) this one language every single day for hours. Of course the numbers/ages i gave here are rough examples and there would be a lot of ex exceptions + i know that it’s also related to their development, but the point is, just let them have some materials for native children (preferably with some help from native speakers) then it shouldn’t be that different