If you do decide to wait until later in the evening to see the dwarves, make sure you don't wait too long. Even though the characters are supposed to be out until 11:30, Disney may cut the line off before then if it's really long. That happened to us last Sept. with the princesses. At 11:00 the line was 60 minutes long so Disney closed it. A wonderful CM took pity on my very disappointed daughter and let us in anyway. But it was a chance I won't take again for important characters.
If the dwarves are super important, I would get in line 5:45-6 pm and get them out of the way. I didn't get in line until 6:30 and waited 90 minutes, losing an entire hour of party time. Even though it didn't work out, our plan was for DH and the kids to eat dinner at Pecos Bill's, while I ate dinner in line, and then he would take them on some rides and trick-or-treating while I waited in line.
A number of families had 1 parent holding the spot in line while the other parent and kids did party things. I think most people will understand and be OK with it as long as you don't have tons of people in your group and want multiple photos.