We had dd help last year and she loved it. It was nice to do it as a family for the first time too. She mostly enjoyed looking at the ornaments and playing with them.
My girls have always helped decorate the tree. Both will help this year. When they get bored, we finish it off. Sometimes we lift them up to get the higher branches and sometimes we just all decorate together. It's a tradition in our family to get new ornaments every year so when they leave and have trees of their own they have tons to get them started. I had three or four big bags when I moved out. We haven't gotten the girls' ornaments for this year yet, but we can give them those later. They've started two little bags for them and do their own first and then help with "ours" (mostly mine).
I am guessing both girls will "help" some. I think two years ago DD1 took ornaments from DH to me or visa versa. Last year DD1 and I did the decorating while DD2 napped and DH was at work.
My 2.5 year old and I decorated our tree just this evening. She loved it! She was so excited about the tree this year and when I told her we could start decorating she dug in and had a blast.
Re: Will your 2.5 yr old help decorate the tree?
DS was 18 months last year and he helped. He hung some of the low balls, and he helped put the angel on the tree. He had a blast!!
The best was some of our bigger ball ornaments, he thought were soccer balls and tried to kick them!!