This is the 441st most frequent Russian word.
детьми
The Russian word "детьми" means "children" in the instrumental plural form.
In this sentence, 'детьми' represents the instrumental case of the plural noun 'дети', meaning 'children', used here to describe the company of the subject 'I' during the action of walking.
Я гуляю в парке с детьми.
I am walking in the park with the children.
Here, 'детьми' also represents the instrumental case of the plural noun 'дети', depicting the individuals the teacher is conducting the music activity with.
Учитель занимается музыкой с детьми.
The teacher is engaging in music with the children.
In this example, 'детьми' (modified with 'своими') again serves as the instrumental case for 'дети', identifying the beneficiaries sharing moment of the subject.