Any times when the word child or children occurs in Mark, it is found in the parallel place of Matthew, and most cases also in Luke.
The admonition
10:15 Amen I say to you, whosoever shall not receive the kingdom of God as a little child, shall not enter into it.
is not in Matthew only in Luke. Also in the story of the Cananite women Mark says with three sentences that the children = Jews have priority, which is only one sentence in Matthew.
Still children does not seems to have special significance in Mark.
Passages with the word child/children: Mk 2:18-20, 7:24-30, 9:14-29, 9:33-37, 10:13-30, 12:18-27, 13:12, 13:14-20