We live in the Chicagoland suburbs and have similar weather as you have in Mo.
So Cal weather is not what we expereince here. If it's 90 during the day here it only cools off by a few degrees at night. In So Cal it can easily be 90 during the day and in the low to mid 60's at night.
We have spent the majority of the past 24 years at DLR during the first several weeks in July. We have also been to DLR in Jan. Feb. Aug. Oct. and Dec.
There have been VERY few nights and early morning where we did not need atleast a sweatshirt or jacket in July. When my kids were little they would usally need long pants and jackets at night.
Another aspect to think about is DL is surrounded by a berm and most of ground level is protected from wind. So you can be inside DL and be comfortable and leave the park to go back to your hotel and be really chilly.
As an adult I'd never go to DLR without a couple of sweatshirts and one pair of long jeans/pants. YMMV.
dsny1mom