i might add think about getting a little camera, be it mirrorless or p&s. my daughter is 15 months now and i've hardly ever used my leica 350/4.8 during those fifteen months despite the fact that it was probably my most used lens prior to her birth. no time and no room for it when i'm out with her. little lenses on my NEX are easy to take with me on lunch at work or out for a walk with my daughter (i've gotten good at one handed shooting). it's not the same, but it allows me to continue to enjoy my hobby despite the massive reduction in time. i'm sure someone with as good an eye as you would have no trouble making fantastic images even with a camera phone. a little camera won't give you more time or daylight but it'll be easier to take advantage of the small little moments you have.
^ Yes sebboh, quite right. And I'm sorry to say AF is often a must for toddlers so this is another reason to avoid NEX+alts for these action pics. I'd take a E-PM1 if budget allows or any goos P&S (I chose Elph 300 because it's *tiny*).
There will never again be a manufacturer daring to produce anything as weird and unique as the 58/1.2. Keep it and have your son inherit it. While waiting for that I'm sure you'll find an occasional use for it. If you havent grinded the retainer ring for the 5D2 dont bother, your son will be using mirrorless stuff.
Get a stylish leaded box if you're lucky enough to have the radioactive version.