MR,
The old rule of thumb we used was "after two sets of true leaves you can move them". Mine started outdoors are almost to this stage in many of the pots. The heuffelii ones are probably the furthest along. Be sure to keep them moist when transplanting as they have very small roots. It's a stage where you can lose them if they dry out. It's a fine line between MOIST and TOO WET however. The later will make them rot. Outdoors this is more a concern than in your indoor conditions.
Good luck with the babies!
Kevin
