How can I use videos and digital picture books to help language development?

The key is not to just let them watch, but for an adult to enjoy it together and respond to the child's reactions with words. Choose clear content suited to their age and development, and use it in ways like: putting a short word to whatever they point at, pausing and waiting for a reaction, imitating songs and lines, and connecting what's on screen to real objects and life experiences. If it's too fast, it's fine to lower the playback speed or for an adult to sing it again slowly. That said, no particular tempo has been confirmed to promote language development. Use video only in a way that doesn't replace face-to-face conversation, paper picture books, hand-play games, play with real objects, sleep, or exercise.