Congrats on your new fish! They're a pleasure to watch as a family when they have fry. I've had a line of kribs I've been working with for the last 6 or 7 years and I never get tired of looking at them.
There's no guarantee they won't eat the platy fry, though chances are good if they're well fed they'll leave them for now. However, since you got yourself a pair, be warned that kribs are vigilant parents and aren't above doing some real violence to threatening fish. If they do have young, they may very well try killing off any platies, fry or adults. How large is your aquarium? If it's large enough, there may be enough room for the other fish to avoid the family, but I would say anything under 30 gallons is going to be cramped for more than the kribs. I have a breeding pair in a 30 breeder with a bunch of endlers and they do them no harm, but there are probably over 20 of them of various ages, so it's hard for the kribs to concentrate on any one fish and catch them.