Moping around is so easy to experience and feel as adults that it's easy to relate to it in children. Let's see... if I'm tired, frustrated, just failed at something, or have been told off, I might start sulking.
Granted sulking isn't a word usually attributed to adults, but we do it anyway. Conversely, what is a definite mark in young children is the stomping!
We see stomping in grocery stores, shopping malls, toy stores, you name it, it's there. What is common is that there is generally something tangible that the child wants. He or she just saw it but it is not in your plans.
So what to do?
The child has just been denied something that they think they are entitled to. Simple isn't it? So rather than say 'maybe tomorrow', or 'not right now', or, heaven forbid, 'go ask your father', deal with the entitlement ahead of time.
Kids want everything they see, but they simply do not deserve to have it so set it up this way, and discuss it.
Establish a plan, an allowance, chores. Do what you know you should, but just don't get around to. Have them set a goal on something and earn money or points toward it. Put a value to each an every object. They have to learn sometime, but it won't do if the child can't feel what the value is.
Start a savings vehicle in house and make deposits to it for chores done, weekly allowances, to bed on time etc. Let the child see and measure the progress. Please DO NOT do like so many young parents and set up a real bank account too early only to deposit grandma's $50 cheques and let the child manage it. Establish a purpose for grandma's money and make it untouchable, if you must have the account at all.
The child's 'in-house account' can be applied, in part, to the next item they really want, provided it passes mom and dad's safety and suitability tests. You can cover half, three quarters whatever you decide, but the child makes a decision and consequently, is no longer flippant in the store.
They will think twice before losing the chance of getting what they really want over the thing they just saw. The result... NO MORE STOMPING OR MOPING AROUND!