Your sunflower seedlings need sunshine. Try moving the babies outdoors on sunny days and bring them inside at night. Do not fertilize the seedlings until you plant them outdoors. Then use a slow release plant food like Osmocote to prevent rapid growth. Let the young plants experience wind and a bit of cool but not frosty weather outdoors in a protected spot to harden them off and slow down their growth. If they continue to grow too fast and become more than 12 inches tall just transplant the young plants into the ground but be prepared to cover them with a cloche or light sheet at night so they don't' get too cold.   The good news is you have a second chance. You can start a few more sunflower seeds now to be ready when all danger of frost is past or just seed directly into the ground at the start of summer. Keep growing, Marianne Binetti