I send thankyou notes in a weird way:
I take pictures of my son USING his gifts, and send them out fairly randomly. Heck... one gift my son got when he was 3, I just sent out another "thankyou card" to my cousin who got it for him... because he STILL uses and plays with it at age 9. (A stuffed frog the size of NewHampshire that he uses as a pillow right now. He was sleeping with this GREAT smile on his fcae and his arm totally hugging the frog... so I snapped it... and sent it out to my cousin).
It's January. If you get the cards out now... you still have a good 2 months (and maybe 3) before you're sending out invitations. Which is MORE than enough space in most people's minds.
You can also include a cute little tag line with it / mea culpa.
- When they say 'they grow up so fast' I didn't realize *I* would be in a time warp, too! I swear this was 2 weeks ago. 9 months later... I just wanted to make sure you know how much we love and appreciate you.
- It's SORT of like a wedding.... yes? With a year to get these out? Darn. No. I didn't think so either. Sending you our warmest, if very belated, best wishes and thanks.
- Alice in Wonderland Rabbit on the back of the envelope (they have those stickers!) with an "I'm late! I'm late! From a very important date!"
etc.
Not that you have to. Most people will just be touched to receive them.