If the children did not send the money with the intention of gifting (giving) it to their mother and making her its owner, but merely for the purpose of covering her treatment expenses, intending to reclaim any unspent amount, then the money does not belong to the mother. Rather, it belongs to the sender(s). In that case, inheritance rulings are not applicable.
However, if the children sent the money intending to give it to their mother thereby making her its owner, and she or someone on her behalf took possession of the money, then it became her property. In this case, the remaining amount should be distributed among the heirs according to the laws of inheritance prescribed by Shariah.
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دلائل:
الدر المختار: (687/5، ط: الحلبي)
(هي تمليك العين مجانا) أي بلا عوض لا أن عدم العوض شرط فيه، وأما تمليك الدين من غير من عليه الدين فإن أمره بقبضه صحت لرجوعها إلى هبة العين ... (وشرائط صحتها في الواهب العقل والبلوغ والملك، وأن يكون مقبوضا غير مشاع مميزا غير مشغول) كما سيتضح.
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.