Our 3 mad one's are aged 5, nearly 4 and 20 months.,
so a bit of a mixture in ages
Overall, they do get on well but i would say that my 2 girls can be a bit bossy with my boy (5 yr. old). The only time i did worry was when our 1st girl puppy was introduced to our boy and the look on his face was just of complete disgust
It did take him a while to accept her but now they are the best of friends. Generally I think it is really lovely for them to have each other and can't imagine just having one dog now.
If you do decide to go ahead with a puppy, please make sure you still fuss and give loads of attention to your Poppy, as obviously you don't want her to feel left out. With the puppy, it is a good idea to take him/her out on their own for walks sometimes and to puppy training classes.
Another option would be to re-home a rescue cocker, which would obviously be closer in age to Poppy. Most of the dogs for re-homing that I have often seen on COL, request another dog to be living in the home too. Something to consider.
Good Luck for the future and I am sure you wouldn't regret having another.