子供が英語を習いたい時はいつでも好きにやらせてあげるって英語でなんて言うの？
今、子供は英語にはあまり興味はないけど、いつか興味を持った時に全力でサポートしたい的なニュアンスです
回答
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When my child wants to start studying English, ill let them do as much as they want
英語を習いたい - want to study English
でも上の文章は始めたい感じがあるので“start“の入れます
Wants to start studying English
（私の）子供
My child, my kid
好きにやらせてあげる - let them do as they like. , let them do as much as they want
When my child wants to start studying English, ill let them do as much as they want
イギリス 
