first of all, what light do you use ? if only a few CFLs just let em stay on, but no need to run a 400W MH, just keep a eye on em and turn it on once they broke the soil
about your low humidity, 21% is very low, IME all under 30% is too low for MJ even in flowering, 40-50% is more like it, and small seedlings and clones even thrive better with up to 80% (never go over 70% in flowering or you risk mold) so you should try to raise it a bit, at least get it up over 30%
a bucket of hot water in your tent/grow room can help a bit, but not more then a few %
misting them a few times a day would be the best way to help the small seedlings get enough moist until they had a chance to grow a decent root net to support em
tho best thing you can do is to invest them +40$ on a humidifier, I did last winter as my RH hit 28%
similar to this
that and some misting will for sure help seedlings a lot even if the ambi RH is as low as 20%