For the Ikea Kira You can just buy an extra set of ikea slats and for them on with a couple of long bits of wood underneath
I put my sensible 4yo in a bunk bed at this age BUT the top bunk was low (1m) AND, although the intention was to put the 2yo on the bottom bunk, he had to stay in a cot with bars til he was 3.5 because I couldn't trust him not to climb up the ladder in the night 🤦♀️so depends on the kids really!