I assume you're referring to Milkweed? I would keep them moist but not wet, letting them dry slightly between watering. Depending on your spp, some, like incarnata, like more water than say, tuberosa, which doesn't tolerate wetness. Best rule of thumb, let your plants "ask" for water, but be careful not to let them dry out too much. Lots of sun and warmth, too.
Good luck c your seedlings; I'm still struggling c mine because of our lousy weather and lack of sun...

yes butterfly weed Asclepias tuberosa, I am so sorry about not saying what my seedlings are.

Tuberosa can tolerate pretty dry conditions. I'd let the soil get barely moist before I watered them again. Are they indoors? If so, remember that before you put them outside, you'll need to "harden them off" , or gradually get them accustomed to the sun, wind, and extremes of temperature they will encounter outdoors. If you google "hardening off", you'll find instructions, I'm sure. Good luck, and thank-you for helping the Monarchs.

Martha