Is pressure inside Jet plane is nearly 1atm when it flying?

The term "Nearly" isn't very specific.

While many aircraft use pressurized cabins to aid the occupant's breathing, the systems don't maintain sea level pressure through the entire flight. Maintaining the equivalent of some altitude between sea level and 10,000 feet reduces the mechanical stress on the fuselage's structure. Most structures, including things other than aircraft, are better able to tolerate SOME stretch and/or flex that be so thick (thus heavy) as to be essentially inflexible. Allowing the cabin pressure to simulate a somewhat higher altitude doesn't stretch it quite as much and allows the aircraft a longer service life without lowering the pressure enough to endanger the occupants.

Regulations vary in different countries, but there are often limits on the time an aircraft can be above 10,000 feet above sea level without cabin pressurization OR all occupants using supplemental oxygen supplies (masks or those nasal tubes). Oxygen and/or pressurization is eventually required for even short ventures above a certain altitude. When the cabin is pressurized the pilots are required to have masks available for RAPID donning in case cabin pressurization is suddenly lost.

Ok thanks,
Oxygen level also important. I accept it.
I have another doubt, that is if pressure decrease, I thing our body expand. Is this correct?

As long as the cabin pressurization equipment is working properly, which it nearly always does, the only thing you will notice is a need to equalize the pressure in your middle ear with the cabin air during the climb and descent. As long as the tubes connecting your middle ear to your throat aren't clogged that can be accomplished by chewing gum, swallowing or yawning (there are other methods). It wouldn't be a bad idea to check with young children's pediatrician to make sure they don't have any health problems that are incompatible with the pressure changes.

Note that untreated congestion from head colds or allergies might be accompanied by clogging of the tubes in your ears.