I wouldn't say Old Orchard Beach is seedy. I'd say more like cheesy. Depends what you want but that whole boardwalk area with the amusement park is cheesy and the general vibe of the OOB area, at least to me, is pretty low brow. I guess for a couple days and to keep your 6 year old occupied it couldn't be too bad. If you go there I'd stay at the highest level place you could. We went once when the kids were little for a long weekend and I would never go back.
When my kids were that age we did a week near North Conway, NH and had a blast. The amusement park
Story Land is made for kids your daughter's age. Can't really go there past age 7. My kids were 4 and 7 and they loved it. It's well run and relatively inexpensive for an amusement park. We went to some of the other amusement parks around there like
Santa's Village, which was about an hour from North Conway. Also did some easy hiking. Plenty of stuff to do around there. The accommodations near North Conway were much better than anything we saw around Old Orchard Beach and the restaurants are much better also.
If you're looking for a beach area you could do much better than OOB. Just my opinion.