You can reach in but be careful when the hormones set in, you might get some cage aggression.
I would assume since the bunny is a tiny baby, right? You're the right person right? Anyway, since its a tiny baby, you probably don't have anything to worry about with hormonal behavior.
I think they say not to reach into cages with the rabbits in there because its "their" personal special safe space, but I think everyone reaches into their rabbits cages.
If there is a way to let the baby out while you clean the pen then maybe do that, so not to stress him out. I always let Ellie out to clean her cage, not because she's aggressive but because her cage is awkward.
Or if you have a box in there for the baby to get into while your cleaning or whatever, then that would be nice for the bun.