Immanuel Kant argued that we do have an obligation to at least sometimes help others, but he famously argued that this duty was ‘imperfect’.
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This means that we often have a lot of choice about how to help others. However, Kant also argued that we must ‘always treat humanity…as an end in itself’. We need to genuinely care about the good of others. So even if we normally have a lot of choice about how to help others, if failure to help on a particular occasion would show that we don’t really care about the good of others, then we would be required to help on that occasion.
Presumably, if you care about the drowning child, you will save him from drowning even if that means ruining your new clothes. So, if Kant is right that we are required to care about others, then it seems that we are required to save the drowning child after all.
On the other hand, if we are not required to care about others, it’s hard to see how we could ever be required to help them, as you claimed.