pstree is useful tool, but it is not a default application when you setup CentOS. So you will need to install it by yum.
You will be able to use pstree after running below:
sudo yum -y install psmisc
Below is an exmple of pstree
# pstree systemd-+-NetworkManager---2*[{NetworkManager}] |-agetty |-auditd---{auditd} |-automount---5*[{automount}] |-chronyd |-crond |-dbus-daemon |-httpd---11*[httpd] |-irqbalance |-java-+-java---67*[{java}] | `-193*[{java}] |-java---157*[{java}] |-lvmetad |-master-+-anvil | |-bounce | |-cleanup | |-pickup | |-qmgr | |-smtp | `-trivial-rewrite |-memcached---5*[{memcached}] |-mysqld_safe---mysqld---120*[{mysqld}] |-polkitd---6*[{polkitd}] |-postgres---49*[postgres] |-redis-server---2*[{redis-server}] |-rsyslogd---2*[{rsyslogd}] |-sshd---sshd---sshd---bash---su---bash---pstree |-systemd-journal |-systemd-logind |-systemd-udevd `-tuned---4*[{tuned}]