Why did He strengthen the bolts of the gates?
Radak: They will never fear lest the enemy come against them. Hashem will guard them like strong bolts - "Al Chomosayich Yerushalayim Hifkadti Shomerim" (Yeshayah 62:10). I.e. they will live without fear, as if there are guards on the walls. Amidst great confidence of Shalom, the gates will always be open - "u'Fitchu She'arayich Tamid Yomam va'Laylah Lo Yisageru" (ibid., 60:11).
Malbim: The enemy will not be able to approach you.
How did He bless her children inside her?
Radak: They will be blessed in the fruit of their wombs and their work, in the house and in the field.
Malbim: You will not need the strength of horses for war, for no one will fight you. You will not need [strong] thighs of man to seek food from afar, for your children will find Brachah, even though they are in you.
