It's normal for minions to take damage, especially the first tier. You don't need to constantly heal them if you don't want to, though. Your summon and upgrade powers recharge very fast, so your demonlings are very easy to replace. The higher tier minions will be sturdier, but they're always replaceable.
If you're still concerned about it, here are some questions: are you enhancing their damage and accuracy? This is the most important thing to enhance early on. Defeating enemies is a form of damage mitigation, and all that.
Are you using personal attacks (which at lvl 6 Demons would be Corruption and/or Lash)? At very low levels, your own attacks do a significant chunk of your damage. This ebbs away as you get more minions, but it's never completely useless. Especially for Demons, since the whip attacks have a pretty strong secondary effect.
Did you skip the upgrade power at lvl 6? You should never skip those, each upgrade adds a lot of power to every tier of minion.
Thermal also has shields that reduce the damage your minions will take. These are just as important for keeping your minions on their feet as your heals. Properly slotted they'll provide a hefty damage reduction to most damage types. I wouldn't call them essential takes by level 6 or anything, but they're something to keep in mind as you level.
There's other, more complex things to consider later, but I think I covered most of the basics?
Oh, and finally...welcome to the game! I also joined back in the day for the Masterminds. They remain my favorite archetype to play and I hope my questions have helped you get started playing them. 😊