The Botanical Garden is nice, as someone posted -- but be aware that if it's a weekend, there can be very long lines for their display.
You're in Lorton so this would be a haul, but the nighttime lights at Brookside Gardens in Wheaton, MD are fantastic. You park and walk through so bundle the kids up, but it is SO worth it; absolutely magical and there is also a greenhouse full of a wonderful train display. Look it up online for times and cost. It really is a gem and we adore going there. It's not far off the Beltway via Georgia Ave. and one very short turn on Randolph Road, so don't let the directions daunt you!
If you do Brookside, be sure to get there pretty early when they open the gates for the evening (probably around 5-5:30??). It does get crowded and the parking is somewhat limited, but again, it's so lovely it's worth it.
Union Station in D.C. also is all decked out. Not any kid-specific activities but a gorgeous huge space gorgeously decorated, and there usually is a very large model train display. Another place people go now is the new National Harbor on the Md. side of the river, across from Alexandria; they have huge themed displays indoors, but do be aware that it's costly. For my money I'd rather go to Brookside -- less commercial.