if you keep her the kittens won't do much until they're almost 5 weeks other than feed and sleep and you can rehome them at 8 weeks so there'd only be a few weeks of dealing with them, and with only 2 of them i'd imagine there wouldn't be too much commotion.
if she's happy enough to let you close to her she'll most likely tolerate being moved ime, i had to do this with a stray that gave birth on my couch and what i did was very carefully pick the kitten up and put it in a cardboard box with a blanket in it and showed her... she jumped in after it and i moved them both to their new loaction where she was happy enough to remain.
my current female gave birth in the toy shelves a few weeks back and when she popped to the litter tray i moved the kittens (she'd had litters before so i wasn't concerned about her taking offense) when i plopped her in with them into their travel cot she proceeded to give birth to another!
you won't need to do anything at all for the kittens for the time being, she'll feed them, clean them and mind them but do make sure she has a constant supply of food and water.
once they're about 4/5 weeks they'll become interested in food and once they start to eat they need a toilet area. 2 tiny kittens, you'd get away with a piece of newspaper on the ground tbh but no harm in litter training them for their future owners... this involves catching one of them in the act and plopping it in the tray... they'll use it automatically after that, kittens are the easiest creatures to house train.
if you give them to the rspca and they're planning on seperating them from mum that means they're going to have to be hand reared, which isn't a great option tbh.
i've had 13 kittens in this house in the last year and honestly, they only make their presense known in the week or 2 before their rehomed and even then they sleep the majority of the day. chances are mommy cat won't go anywherre far for days and when she does for the next few weeks it'll only be quick trips out so she won't need constant access in and out, could you put a cat door anywhere in the garage, or put crates up to a high window or something?