Because most kids have the whole week of Thanksgiving off school, both DLR and Legoland will be busy. In my opinion, the days after Thanksgiving at DLR are not as bad as the days leading up to it. Mon and Tue last year were 10 out of 10 crowd levels on Touring Plans (those were the actual "what they saw" figures). A friend of mine was there on those days and she confirmed that it was crazy. I would avoid those days if possible.
We were there Fri to Mon right after Thanksgiving last year and, even though it was crowded, it was still manageable if you go in with a plan. Getting there early is always key. I think Saturday was the busiest day, but we spent most of our time in DCA and it was better than DL. That was the day we rode RSR 3x in one day! We hopped over DL around lunch time and I remember the line for Pirates wrapping around and up the bridge so you KNOW it's crowded when that happens! The Sunday after T-day was definitely the best, but I feel that way about all Sundays in DL. They are such quiet mornings and I don't recall the afternoon being too bad that day either, maybe everyone heading home after the holiday made for lower crowds that day.
So, based on your dates, I would suggest going on Sunday, the 22nd. I will be there on that Sat and Sun, hoping that it's not too crazy. If you all get to DL for rope drop on Sunday then you will have a great morning.
I think this year will be more crowded in general because of the 60th, but like I said, it's always do-able if you get there early, have a plan, and use FPs to your advantage. Hope you are all able to visit during the holidays. It's so beautiful that time of year!