One definition of symbols is that they are anything which is given a meaning that is different from its literal one. As such, almost anything could be a symbol, but typically the application of symbolism in literature is reserved for inanimate objects, nonhuman creatures, or humans who are acting upon the protagonists.
There aren't terribly many symbols in "The Veldt", at least not ones with a clear and definite meaning beyond their immediate relevance to the theme that technology has corrupted the Hadley children. For example, the rocket trip to New York City that they were denied probably doesn't carry any inherent symbolism, it simply serves as a good example of a small thing blown out of proportion by the entitled and linear minds of the children.
The contents of the nursery are probably a symbol. David McClean, the psychologist, says it is "filled with hatred" and that he could feel it "coming out of the sky", indicating the sun in particular. Thus both the sun and the lions may represent oppressive, unassailable hatred and power, which the children feel towards their parents.
The scene and setting of Africa itself may represent the absence of civilized influence, or the reversion to "wild" instincts and resolving conflict through violent confrontation. Africa is also where humanity came from, so it may represent the children reverting to an animalistic way of life, because the technology has prevented them from developing into humans.
The nursery itself may also be representative of the archetypal Cave, or however one wishes to name the lair of the enemy which represents a challenge to the archetypal Hero. The Hadleys begin to treat it almost as a separate living thing, and going to it carries a sense of foreboding and danger; it requires a mental commitment and effort as opposed to, say, just going to the kitchen.