At this level, I consider your child as an advanced player. S/he can do damage to any player, even GM, if that player has made a big mistake, such as blundering away a Rook. S/he won't be swindled away his/her entitled win.
Your child already knows all tactics, and most basic endings. S/he knows many openings or has seen many openings. S/he still needs to learn deep calculation, and specific endings, such as rook endgames. But the most s/he needs are strategies, positional knowledge, and opening lines (opening repertoire). S/he may be self-sustainable, self-teaching him/herself everything. Working with a titled coach, GM or IM, will be more efficient and effective. This is a time that requires serious study.
Your child should be able to reach this level in 3 years at most. If not, s/he may want to consider chess as a total fun hobby.
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