Here's the thing. This baby is a super blessing to everyone because he is an answer to prayers. Everyone that knows you that has faith said at least one prayer for him while he was in crisis.
SO they are going to want to be part of his celebration. Let them bring gifts and bless him back.
He will grow into the toys when he gets there. The toys that are for 9-18 months are made to grow with them. I think that if I got an invitation that said "we don't want your toys" I'd be annoyed.
SO, that said. If you really want to do this then you'll do it anyway so why not word it like this.
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"We know that everyone loves XXX and want to show him your love we are letting you know that IF you'd like to make a donation to the Nic-U unit that was such a blessing to us when he lived there instead of a gift to XXX we'd like to thank you".
The doctors, nurses, and staff at XXX Hospital Nic-U were kind and helpful and they went out of their way to make sure XXX did the best he could so this is our way of thanking them, giving to their program so they can help even more families.
Here's the address for donations to be sent."
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That might make me feel differently about not giving him a gift but still, I might make a donation and give him a gift too.