I knew asking in this section would get me answers! thanks everyone!
archer1960 wrote in post #16442083
I'd say the 24-105 would be ideal, but the 18-55 would likely be good as well. Both will likely need some additional light, so make sure you bring your flash.
Flash is in the bag.
DreDaze wrote in post #16442157
i think maybe the newer sigma 30mm f1.4Art...
take your 18-55mm...use it first, and see what focal lengths work out for you
you say you wouldn't go over 50mm, so the 24-105L seems like a waste, you could get a 17-5X f2.8 stabilized zoom instead
As much as I'd like the 30mm ART, I think the 24-105 or a 17-55 would see more use in general based on what I shoot typically.
davidmtml wrote in post #16442169
If you don't have a flash, I think that will make a bigger difference than any lens will.
Oh, yeah, flash was already planned. I have quite a number of them (upwards of 20, but only actually use about 4)
MalVeauX wrote in post #16442187
Heya,
I think the 30~35mm range with a flash bounced from the ceiling would do you well. Your kit lens would do this just fine. I wouldn't use this as a reason to buy new glass unless you just want new glass for a lot of other reasons.
Very best,
I'm always looking for a reason to get new glass! Aren't all of us to some degree? 
Russo09 wrote in post #16443035
Tamron 28-75 2.8. Great sharp lens that I think will fit your needs, and doesn't break the bank.
I'll be looking into that. Nice range, and fast.
BrickR wrote in post #16443057
I rented the Sigma 30 1.4 Art. Really good lens. Flash will have more impact on your photos than a new lens will, so that would be choice #1 for me. Then would come a new lens...although it is obvious you are trying to find a reason for a new toy

You read between the lines.
xarqi wrote in post #16443079
To echo some of the suggestions above, use your 18-55 IS and bounced flash.
Will do
hennie wrote in post #16443159
FL will be your main selection criteria as these kittens will (there will be exeptions) sit still most of the time.
I made pictures of kittens in an asylum before and used my 24-105 on a 40D.
It just depends on how big (or small they are) and what kind of photo's you want.
Getting to close or using flash might scare them off, remember that some will have a traumatic experience with people.
For head only I was mostly shooting around 70mm FL and whole kitten between 40 and 50 mm.
Great stuff, and I look forward to helping these little guys get a new home. You are echoing what I was thinking, that 55mm may be long enough, but I may need a tiny bit longer.
hennie wrote in post #16443182
Shot at 50mm / F5.6 could get very close on this one.
Some of the kittens got a new home and their chances of getting one increased with a nice picture.
So regard it as hobby time very well spent.
I'm really looking forward to it, and they're even allowing my 12 year old daughter to be my 'assistant', so she's been bouncing off the walls! We're both animal lovers, so this will be a great rewarding experience for us both.
Thanks again everyone for your advice!