Oh, NannyKisses, my heart aches for you! How awful to feel as if you might lose contact w/ your GC if you don't send gifts, etc! I realize it may be better than if you weren't "allowed" to send gifts/have contact at all, as is the case for some GPs. But, IMO, it's sad to even have to make this comparison.
I'm not sure if/why the gifts have to be clothes. Wouldn't it be all right if you sent some toys or books sometimes? And do the gifts "have to" be expensive? Even if clothing, wouldn't it be ok for you to send some attractive, fun, but less expensive items?
As for the money for the uniform, this is a tricky one. I definitely think D was hinting when she told you about this. While I don't think you should have offered to pay, given the circumstances, I understand why you did. I probably would have done the same. And I understand your feeling that you can't back down now that you offered. Years ago, MIL reneged a couple of times on financial help she offered. Yes, she had a right to change her mind, but I hated it, as we had already made financial plans around the offers. DH and I didn't CO her b/c of it, we didn't do things that way. But IDK what your D might do, and I know how important your contact w/ your GC is to you. I would go through w/ what I said, but avoid making such offers in the future.
Oh, but then there's the problem of SIL. I think you have to assume the email is from him, it's not your fault if D made a fake account. But I would email back and ask if the offer is ok w/ him before I gave the money.
Clearly, there are some differences between D and SIL over how much help to seek/accept. Unfortunately, this has impacted your relationship w/ your GC. I'm so sorry about that.
The weekly gifts and phone calls are very likely a compromise between D and SIL. ("Ok, if your mum wants to give, then let her give to the kids, and they can ring her up to thank he, but no more than that.") I don't think I could bear to stop if I were in your shoes, but I would cut back on the expense of the gifts. I hope you continue to enjoy those conversations w/ your GC.
Meanwhile, I agree w/ others about building up your life outside of your relationship w/ D and her kids. IMO, we should all have more parts to our lives than our AC and GC. And as others have noted, you never know when D and SIL might go NC, period, over some issue, real or imagined. So please make sure you have a life beyond your relationship w/ them.