Proud that his own were firm and strong.
Line 25: "He guided the trembling feet along,"
Line 26: "Proud that his own were firm and strong."
" He guided the trembling feet along, "
(He carefully leads the woman’s shaking feet across.)
" Proud that his own were firm and strong. "
(The boy feels proud that his own feet are steady and strong.)
Overall meaning: He carefully supports her trembling feet while feeling grateful for his own strength.