If the person cannot, you'll be greeted in another fashion. Speak directly to the person in the wheelchair rather than to any companion. Look at the person when you talk, rather than at the wheelchair. Sit at the person's level if the conversation will be an extended one. Remember the wheelchair is part of the user's personal space.
People using wheelchairs realize children are curious.
If a child asks a question you feel is embarrassing, don't chastise the child. Let the disabled person handle the situation. A polite offer of help is acceptable if the situation warrants it. For example, many wheelchair users find opening doors awkward. You'll notice disabled people also use them as part of their routine conversations.