I would feed both hays; timothy topped up with alfalfa. Alfalfa is strictly spoken not a grass, but a legume. Timothy hay is the best hay for a smooth digestion.
Please remember that your little ones are now eating pellets fully and that there is a good deal of calcium in the pellets, too! Start gradually limiting your pellets when the weekly growth rate starts slowing down ata few months old.
Tegan hasn't had any alfalfa since she was about 5-6 weeks old, and she certainly hasn't suffered!