"arab" means: billion (sense 1); 1,000,000,000; or with Indian digit grouping, 1,00,00,00,000.. This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.