I need to build a toy for young children (over 4 yrs) and it must have
at least one hydraulic or pneumatic moving part. Could you please send
me ideas on how to build a toy with these requirements?

What about a jack in the box? A toy crane or construction vechicle (eg
dump truck)? It would be helpful were you to include more details,
such as budget limitations, complexity (what you feel comfortable
building), size limitations, what the hydraulic or pneumatic moving
parts would be powered by?

A simple pump, like an old farmyard pump, the water flowing into the
basin to be pumped again.  The kids will love playing with the water. 
Mom might be less pleased.

A little more ambitious  - and I don't know if this would work - 
could be a little motor.  The wheel would be turned by a rod from the
piston, driven by water  (maybe avoiding leaks by having a piece of a
balloon line the cylinder (and replacable).  The water would be driven
by squeezing an old plastic toothpaste tube or the like, fastened to
it by a flexible plastic or rubber tube so that the pumping does not
jerk the machine, even long enough so that the pumping could be out of
sight under the edge of the table.
You would probably have to load the thing by submersing it all in
water, with the piston at the water end of the cylinder.

It will require some coordination to time the squeezing with the rotation.
I am hoping that the plastic t-p tube will want to return to its round
shape after squeezing, thus drawing the water back out of the
cylinder, reinforced by the weight/momentum of the wheel's turning. 
Probably would be good to have a little air in the system as a
cushion.
The cylinder should be rather long (another t-p tube?), so that the
timing is easier (low rpm).

Who knows, maybe the thing would work with just air.

Myoarin's Magic Motor!

Maybe use a squeeze bottle of the appropriate size instead of a t-p
tube, but one the kids can squeeze.