#1 - You don’t need to change your child’s name so your other child can say it. As I’m sure you’ve seen with your first one, kids are amazing, the things they can say at a young age.
#2 - You really intend to simply begin calling your child something else based on his age?? That’s so foreign to me, but hey, if it works for you, go for it!
#3 - I hate to disappoint you, but no matter if you choose [name_f]Addy[/name_f]/[name_f]Addie[/name_f] or [name_f]Della[/name_f], [name_u]Theo[/name_u]/[name_u]Teddy[/name_u] might just call her [name_f]Banana[/name_f].
I would call her [name_f]Addie[/name_f] or [name_f]Lina[/name_f] but [name_f]Addie[/name_f] might be easier to pronounce for her brother.
I voted [name_f]Addy[/name_f] 
Of these, I’d probably go with [name_f]Della[/name_f]. I’d never thought it looked terribly cute (not like I think [name_f]Ella[/name_f] and [name_f]Bella[/name_f] do), but I love the sound of [name_f]Della[/name_f]. I like [name_f]Addy[/name_f] (prefer this spelling), but if I ever used [name_f]Adele[/name_f], she’d probably get the nn [name_f]Della[/name_f]. :). I would love to meet an [name_f]Adeline[/name_f] with the nn [name_f]Addy[/name_f], [name_f]Della[/name_f], OR [name_f]Adele[/name_f]. :). I think it’s fine if he calls her [name_f]Addy[/name_f], and you call her [name_f]Della[/name_f], though.
I would stick with [name_f]Adeline[/name_f]. Your son will come up with his own version of her name. I would use something like [name_u]Lennie[/name_u].
You’re probably right about #1. [name_f]Adeline[/name_f] started pronouncing her own name quite clearly and correctly from about the age of one and a half. It will be interesting to see what [name_u]Teddy[/name_u]'s spin on her name will be 
In regards to #2 yes, I do intend to stop calling him [name_u]Teddy[/name_u] eventually (as for a particular time frame, it will just be whenever I feel it’s right) because I really can’t see myself calling him [name_u]Teddy[/name_u] when he’s a 30 year old man, lol! It feels to me more of a cutesy pet name for a baby/toddler/child. I know of a lot of mums who have stopped calling their kids by their cutesy baby names and moved onto a more age appropriate version e.g. Myself. I definitely don’t get called “[name_u]Angie[/name_u] Pangey” anymore! But hey, it’s up to him. If he likes [name_u]Teddy[/name_u], I’ll happily continue for as long as he will let me.
And #3 - well, if [name_u]Teddy[/name_u] decides to call [name_f]Adeline[/name_f] banana, that would be the best!
That’s so cute! Quite a few people have said the same thing. It’s a very good point that I never really considered. I can’t wait to see what [name_u]Teddy[/name_u]'s spin on her name will be! I think I’m also at the stage where I feel like using a nickname every now and then, particularly in a bid to get her name out that little bit faster e.g. “[name_f]Della[/name_f], don’t poke your brother!” 
I love [name_f]Della[/name_f]! It’s so cute and less expected than [name_f]Addie[/name_f]/[name_f]Addy[/name_f]. I also think [name_f]Addy[/name_f] seems really–almost too–close to [name_u]Teddy[/name_u]. Not a bad thing, but just something to consider. I do prefer the [name_f]Addie[/name_f] spelling to [name_f]Addy[/name_f] anyway. [name_f]Lina[/name_f] and [name_f]Ellie[/name_f] are other cute options you could get from [name_f]Adeline[/name_f], but my favorite still remains [name_f]Della[/name_f].
I love your kids’ names, though, btw:) They’re so classic and charming.
Personally I don’t think you need a nickname for [name_f]Adeline[/name_f].However, my favourite nickname is [name_f]Addie[/name_f]/[name_f]Addy[/name_f].
[name_f]Ada[/name_f] and [name_u]Teddy[/name_u].
Thank you so much! Classic and charming is definitely what we were going for, so I very much appreciate the compliment xx
Go with my name - [name_f]Addie[/name_f]!