given that you are on the 2nd floor, i doubt the ants are coming in from outside, you must have a nest somewhere nearby. ant powder will only kill off the ants outside of the nest, and keep them at bay. only way to get rid properly is to put down proper ant killer, like nippon. the ants eat it, take it back to the nest, make food from it, and it gradually spreads through the nest killing it off. beware though, first, in order for this to work, you need to live with the ants for a few days, don't kill them off with ant powder otherwise, not enough of the any killer will make it into the nest, which it needs to do to kill off the queen. and secondly, the life cycle of a nest, is something like 3 weeks for eggs to hatch. what will happen is the nest will die down once the killer takes effect. but roughly 3 weeks later become active again as the existing eggs hatch, once this lot is effected by the killer, the nest should be gone as there is no queen now to lay any eggs.
i know all this as we have 2 nests in our kitchen last summer, we had to remove the floor, concrete included and i did a lot of research into ants nests!
you can get child safe killer traps, the are like plastic domes, kids can't get into the poison but the ants can.
good luck!