If your LO is on her back and seems to want to roll over, you can put your finger in one palm and when she grasps it, go ahead and give a little pull to help her roll over so she can see what it feels like. Sooner or later she'll get the hang of swinging her legs over to do it herself and she'll need less and less help. Putting a toy on one side of her, then helping her roll over to get closer to the toy might help motivate her to want to do it more herself.
You can also hang things over her if she needs a break and wants to be on her back for a while. Tying a ribbon of a balloon on her wrist for her to bounce the balloon with might keep her fascinated for a good long time, although you DO have to be there with her when she has a ribbon so it doesn't end up around her neck.
At six months she might also be looking to have more Mommy time, or to get into solids at the table/high chair. If you suspect Mommy time and have a good strong back, a carrier might give you both a break - she can see what you're doing and you get a break from the whining for a while.
And for tummy time, if you put him on his tummy on a fairly low-friction surface, like those foam puzzle mats, you can put your hands behind his feet to let him push off and get the idea of moving forward. Even if you only do it for a few minutes at a time, he'll get used to longer and longer tummy time and eventually will probably prefer it since he'll be able to get some motion on his own, to get to places. Be warned: once he gets used to moving, he won't want to stop!!!!
The good news is that once he gets the hang of doing stuff himself, you won't NEED to be with him constantly. Heck, in Natalie's case, with the older one, I CAN'T be with her all the time, or Josie would get lost in the shuffle (as if she'd let THAT happen LOL). There are times when I put her down and let her motor around and I'll check in on her from time to time to make sure she hasn't gotten into something she shouldn't and then go back to doing laundry or making supper. When she DOES need me, she really lets me know, and then I take a break and get some dedicated Natalie time when I can.