At which wanikani level can I watch childrens anime? And 2 other questions

There is a bit of a conflict between using WaniKani (reading) and aiming to understand anime without subtitles (listening).

Do you plan to do any reading of Japanese books or manga? If so, you may want to check out the Pokemon and Cardcaptor Sakura manga.

Regardless of where you begin, starting out will be a struggle. You need to learn words, you need to learn grammar, you need to encounter those words and grammar in context (and in different contexts), and you need to encounter them enough to build up pattern recognition.

Reading won’t help with listening, but it can help with getting used to vocabulary and grammar that you’ve learned. If you want to try reading for this purpose, I recommend checking the list of completed books read in the Absolute Beginner Book Club and checking whether there’s anything of interest there and trying to read it (after learning some basic grammar) while utilizing the old book club threads.