It is appropriate to explain a few technical terms before dealing with the question:
1. Great scholars have defined the term 'saghir' (minor) as such: Saghir (minor) refers to a child who has not attained puberty. Such a person is not eligible under Shari'ah law to have discretion over his property. Thus, he cannot sell[1] and gift his property, nor can he sell and lend it to someone or make any other deal involving the property, even though such transactions may be in his best interest and the child assumed to be best capable of discerning right from wrong, good from bad.[2]
2. The authority to have discretion over a minor's property rests with the child's father and paternal grandfather. In the absence of these two, whoever is appointed as guardian (by father and paternal grandfather) would have the right of discretion over his property. If the child has no father or paternal grandfather and none has been appointed for such purpose, guardianship over the minor rests with a qualified Mujtahid. As for mother, paternal grandfather, brother, paternal and maternal uncles, they would not have guardianship, unless one of these people is appointed as legal guardian. In the absence of a qualified Mujtahid, guardianship rests with just ('adil) believers (momeneen).[3]
3. The guardian or the protector of the minor must use his property moderately; he should not use it lavishly and extravagantly or narrow-mindedly.[4] In case of extravagance, he will be liable.
When it comes to your question, there are a few points that you need to keep into consideration:
1 – If your mother is your guardian (on account of a will made by your father or paternal grandfather or because he has been appointed as guardian by a qualified Mujtahid), her discretion over the children's property is allowed under Shari'ah law.  Perhaps, she might consider something for children which best meets and fits their interest.
2 – If your mother is not the children's guardian or her way of handling things is not in the best interest of the family, you must, with all due respect, talk to your mother to let her know what she should do actually. Make sure not to do anything disrespectful to her.
3 – You should know that your mother might have inherited something from your father which she has the right to use.  Perhaps, she might be paying for your uncle's expenses from those properties.
4 – In any case, we advise you to respect your mother and give her what she deserves as a mother because Imam Sadiq, peace be upon him, said, doing kindness to parents is, like doing jihad and offering prayers in the prime time, one of the best deeds.[5]

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