Bad Manners and Brimstone

Yes, I think you should. You can slip in a mention of having received her delightful thank-you for the first item, just to make clear that you aren't accusing her of not doing her social duty. At a time when mail and delivery services are all over the place, it's 100% legit and reasonable to warn someone to expect a parcel, and ask them just to send an email as soon as it arrives safely (rather than a lovely letter which might take just as long to reach you).

My DH ordered a present for me from the USA in what should in ordinary times have been plenty of time for my birthday; it took four weeks for the carriers to move it between Albuquerque and LA, and another fIve days to arrive at our local post office. He watched its progress on the carriers's site so at least he knew it was on its way, however slowly. But if he had ordered it for some other family member at another address, and hadn't had a tracker site for it, he would have been left wondering if it had arrived weeks ago and the recipient just hadn't bothered to send thanks, or had sent them by snail mail and that response was also stuck in the post.

Yes, kindly say something. The package can get lost, delivered to another address, or stolen.

Last year, I ordered some purple plaid linen (end of the bolt) and it was marked "delivered" but the location it was "delivered to was some five miles away (wrong intersection).  I contacted the post office by phone twice, ran in to the usual mail man at the mailbox & mentioned it again (after I'd contacted the company about a smaller remnant that was the last of the fabric that they had - no more available - but not quite enough for the project "as planned").  The usual mailman went looking around (I told him were it had been delivered - intersection of two other streets) and he tracked it down and it was in my mail box a day or two later.  I tend to try to stay on top of things that I'm expecting...especially when it is something that is not still in production. 

I agree that you can email. I'd go with a "Hi, was checking status of the order from kids store. I see that one was delivered on 26 May but the other items is on backorder. I'll let you know when I get an update on shipping schedule so you can be on the lookout for it."

Ordinarily I'd say no, but in this case you did get a nice note from her, so it gives you some breathing space.  I'd still be super tactful about it, though.  Referencing her sweet note for the other gift is a good idea, I agree.  Maybe something like, "Let me know when the personalized gredunza arrives -- I hope you like it" or something like that.  You'll find some way of making sure that it's clear you aren't chastising them for not writing sooner.  It really hasn't been THAT long, anyway.  Maybe, "By the way, I got a notice that the personalized gredunza was delivered, so if it wasn't, please let me know so I can follow up."  Not mentioning WHEN you had been told it was delivered might help a bit.  And for sure I would include this as an oh-by-the-way in a longer message, not a message only about the gift.

As others have noted, sometimes there is no enclosure saying who a gift is from.  My daughter lives overseas and just set up a registry; she is having her baby gifts sent to our home in the US for us to bring when we come.  I was surprised that two of the four packages that have come so far had no enclosure saying who sent it.  She is able to find out from her registry.  In both cases, they were separately shipped parts of orders; the other parts had come with an enclosure.  One time one shipment was from an independent retailer, the other time both were from Amazon.  So it does seem that it is becoming more common for mistakes to happen with enclosures and delivery addresses, perhaps because the merchants are getting overwhelmed right now when everyone is ordering instead of shopping.