I am newbie here. I am planning to buy a small camera for family photography and need your suggestions. I am getting one Canon 700d with 18-135mm lens (Shutter count is 3000) and it's 3 years old vs new Nikon D3400 with 18-55mm lens (2 years warranty) at same price. Which one should I buy for better photo quality in indoor and outdoor functions.
Which camera and lens combination is better for casual use as inferred from the question is probably irrelevant. Their performance in most respects should be close enough not to make a huge difference.
However both combinations have a clear edge each.
The Canon has a more versatile lens with much greater zoom range. This will be a blessing outdoors.
The Nikon camera's image sensor scores significantly higher than the Canon's in low light ISO performance. This is nice indoors. However the Canon is not terrible, both are way better than any compact camera.
As far as I can tell both lenses have image stabilisation which really help hand holding in low light. So no one have an edge here.
Similarly both lenses are f3.5-5.6 so no big difference in aperture. Depending on how quickly the aperture reduce while zooming there might be a slight advantage in any direction, probably in Canon's favour.
If there is no expectation to ever buy another lens I would consider if slightly better low light performance or more zoom reach is most important and decide based on that. I would probably choose the Canon here but this is personal preference.
If you might possibly buy more lenses you should also consider the lense ecosystem for each mount.
The advantages of Nikon are the possibility of using many old manual focus lenses without adapters, these can be cheap but good if you can live with manual focus.
The advantage of Canon is that all Canon lenses since the EOS system was introduced in 1987 are autofocus lenses and should just work, similarly some of these old Canon lenses can be rather cheap.
For modern lenses I would not consider either mount to have an advantage but depending on exactly what you plan to shoot there might be a lens from some of the systems you prefer which might weight the decision. (The Canon EF 17-40mm f/4 for full frame in my case, so not relevant for these cameras)
Personally I prefer Canon for there lens selection, especially older EF lenses even while Nikon is ahead in ISO performance. My reasoning is that it is cheaper to upgrade one body when Canon catches up than buy what I perceive more expensive lenses for Nikon (and deal with F mount version incompatibilities).
So given this I would choose based on the lens rather than the body making Canon the choice for the more versatile lens.