I would also agree with Swallow Falls. It is a wide flat path and boardwalk getting to Muddy Creek Falls (with nice overlook area). The loop from Muddy Creek to Swallow is an easy hike but does have rocks and roots that you need to be able to climb over. My 70+ mom and 4 year old have done it.
Swallow Falls also has a nice open area with picnic tables and a playground that we like to bring lunch and hang out.
Deep Creek Lake State Park has a discovery center which is pretty cool and our kids love. There are several playgrounds, picnic tables, and a short easy hike right next to discovery center and it has a big parking lot, so easy in easy out.
We will be up taking advantage of the extra long HoCo school weekend