This resembles some flat designs, but is not flat. Shades and gradients represent something 3D. Is the work intended to be flat - that should be known, if one wants to decide have you succeeded in keeping it flat. How well you have succeeded in your design, that is different than how flat this is.
If the purpose was to show that LEGOs are fun, then use perspective, gradients, shadows, highlights as you like and the customer, too.
If I imagine myself to be the customer or employer, I could politely say that "maybe all things should have 3D tricks equivalently, because I didn't order 3D Charlie in flat world - just for the balance!" But it's as possible to get "Nice work! I was afraid that I must pay licence fees to Lego Co. It's a big relief to find that you found this approach." I should know, what is the original goal to conclude whatlike comments are probable.
Flat design is a couterattack against all kind of smaller and larger details that try to make an illusion of three dimensionality for decorative reasons. Flat design supporters believe that presenting everything as flat
- more easily quides the focus to the content, not the appearance
- is more easily adaptable into different screen sizes and user interfaces
- is artistically as demanding and valuable as 3D imitating designs
Flat design is an approach to represent things. It is: Representing the current space of observables choises and the messages without 3D imitations.
A Flat Design work should be based to a decision to be flat and the originally wanted content should be limited to those that can be represented as flat.
Flat cannot hold objects which need 3D imitations to be instantly seen, what they are. Considering this: LEGOs, clothes and hair cannot effectively be represented in flat if the idea is to show their 3D forms. Some other aspects can well be represented as flat, for ex. there is a big bushy brown hair, the boy has fun with his bricks and there are plenty of them, even the wall is a big brick.
Thus, if you're going to keep it flat, remove
- all those compositional tricks that can be interpreted as "something is further and something is nearer".
- the intent to keep 3D forms instantly visible
All presented should be possible to be clipped of coloured papers that are piled onto the table - maybe overlapping, but definitely not the narrow side downwards.
Gradients can be acceptable, if they are not making an illusion of depth. Unfortunately in objects from the real world that is in watcher's eye, so avoid gradients on clothes, hair, toys etc..
Conclusion: Have all the 3D tricks that you like, if nobody wants you to keep it flat. The final impression should be the one that was the purpose. The limitations of the final output media should be taken into the account, too because you may be considered to be the expert on this.