In this situation, are you required to use a sound signal to alert children that you are approaching the pedestrian crossing?

Why?

The presence of children near or on the crossing does not itself require you to sound the horn. You should slow down, take particular care and give way to pedestrians entering or already using the crossing. In a built-up area, the horn may only be used when necessary because of immediate danger.

art. 26 ust. 1 oraz art. 29 ust. 1 i ust. 2 pkt 2 Prawa o ruchu drogowym