A few thoughts:
1) Swallow Falls - Definitely pretty with the multiple waterfalls and offers one of the easiest hikes. If you do a full loop, there are parts that may be difficult for the very youngest and oldest (e.g. over by the small ‘waterfall’ where you have to basically climb up a large rock to get past that part and when walking along the river to get to the large falls). However, the youngest/oldest could easily just skip the full loop and instead walk back/forth to get to each of the falls using the easier paths. There’s a lot of evergreens for the tree canopy so I’m not sure how much color will be there.
2) Another easy trail (although somewhat boring compared to the falls) is Lochlynn Heights Wetlands Trail which is down near Oakland. It’s basically part wooden ‘boardwalk’ through a wetlands (with places to stop to read signage for wildlife) then an easy walk through the forest. That’s near Simon Pearce Glass Factory which the kids may like to go see glass being blown (and adults may like the showroom to buy something).
3) Deep Creek State Park has a lot of trails (https://dnr.maryland.gov/publiclands/Pages/western/DeepCreek/Trails-and-Trail-Map.aspx) but may prove challenging for the oldest. Beckmans Trail is one of the most popular and fairly easy. You’d see good colors here along the way.
4) Herrington Manor State Park is another good option (similar area as Swallow Falls) https://dnr.maryland.gov/publiclands/Pages/western/HerringtonManor/Hiking-Trail-Maps.aspx - there’s a small lake there which is popular in summer, cross country skiing trails for winter, and some good hiking trails. We hiked it 3 winters ago and don’t remember it being hard (there’s several trails too).
Have a great weekend! It should start being a bit less busy there since Autumn Glory was last weekend until the more popular winter weekends so a good time to visit.