How old are they? My one daughter likes to eat the sprouts (but the other one doesn't.
With my kids, they appreciate it more when it's veg's they like. So I always plant cucumbers, carrots, and potatoes in my older daughter's part of the garden and my other daughter gets peas, tomatoes and beans in hers. Radish IS great because they sprout right away and mature in 30 days, but they don't like to eat them.
So. What do they like to eat?
If its warm enough, you could try one bush bean in a cut off 2L soda bottle. Soda bottle containers are great because they can see the roots. One winter, we grew a pole bean plant in a sunny bedroom window and harvested a handful of pods. Younger daughter was 6 or 7 at the time and she happily harvested each pod and asked me to cook it. I have grown a "volunteered in a overwintering container tomato" every winter and she would eat the little tomatoes as they ripened. Finally decided to get serious about it this winter.
This time of the year, you could start spinach, chard, cauliflower, cabbage, kohlrabi, lettuce, etc. and plant them out a couple of weeks after the ground thaws. You could plant peas directly in the ground around 2nd week of March or when the ground thaws.
I know you said from seeds, but for growing from plants, if they like strawberries the kids really appreciate growing them. When I buy bareroot crowns on line, they ship them for planting around first or 2nd week of March for my area. Probably similar schedule for Philly area.