I have lived with cats my entire life and made efforts to observe feline behavior my entire life..
Despite their reputation as "loners" cats DO greatly enjoy the company of another feline.. It encourages play, exercise, and does help keep them from being bored.. That said, it is not necessary for their happiness..
I recommend definitely getting a kitten.. I'd go under 6 months old. That is puberty time.. anything older is, as far as the CAT is concerned, an adult, so to speak.
The younger, the quicker your older cat will ajust, because the less it will be "threatened" by the newcomer.. I don't usually recommend people with an adult cat to get another adult cat.. It just doesn't always work out!! An adult cat adjusts to a kitten SOOO much more easily!! To explain why I'd have to go well into feline "psychology" and heirarchy issues.. I don't think that's necessary here.. just take my word for it.. LOL..
WHAT TO EXPECT
Generally when you bring a new kitten home at 8-15 weeks of age, your adult will hiss, there will be some hissing/spitting/screaming, maybe even a bit of slapping, but I promise the big cat won't actually harm the kitten.. Within a couple weeks it should be tolerating the kitten's presence very well, and possibly even playing with it!!
Remember.. spey/neuter before 7 months old.. if your other boy isn't fixed, do it before getting the kitten..
So, in summary, definitely do it.. get a kitten, not adult cat.. don't be surprised/concerned by their attitudes toward each other when they first meet.. within a few weeks, they'll be friends.
male or female doesn't matter much, but SOMETIMES, a cat will accept a cat of the opposite sex just a TAD more easily than one of the same sex. But it really doesn't matter much..
Spey and neuter before 7 months!!
any further questions, feel free to email me..
I didn't mention young children.. No reason why that should make a big difference.. Just keep in mind that a kitten is more hyper than an adult cat and hasn't learned to "watch it's claws" while playing.. If your children are old enough to be taught how to be careful of the kitten's claws, great.. very young children I recommend just not letting them play with the kitten too much.. There's a big difference between a big adult cat that is calm and doesn't mind a 1-year-old patting it, and a kitten with sharp claws who is "wound up" and thinks that any movement is an invitation to play!! Just keep this in mind and supervise/teach accordingly!
27 years experience with cats.. have gone through this (adding new kitten to the household) MULTIPLE times.. NEVER had a problem with the adult cats accepting a kitten..