People like to feel kind and helpful.
And people also like to feel relieved they've got rid of their unwanted tut ...
So you're the perfect solution! Unfortunately it's got unmanageable.
You'll need to help your Mum say 'no' for you, as she's ahead of you in this awkward chain, and to be honest, she's part of the problem. Is she also ND, and finding it difficult to say no?
Keep repeating 'Mum, we don't need any more xxx. In future if anyone offers it to you for us, please tell them no thankyou, she's fine now, she's actually got too much, and say please could you donate it to (local charity, etc).'
And stress to her that 'At the moment I have to cope with disposing of things we have no use for, which upsets me and takes time away from caring for DC'.
You may need to refuse the next bag and ask her to take it back to them to strengthen your message.
Also send a kind, polite message by email or messenger etc to the donors saying 'thankyou so much for thinking of us. However we're at the stage now where people have been so kind, I'm needing to dispose of items! So please could you donate to your favourite charity instead?'
Hope some of that will be helpful.