No, it's unlikely to be worth it from a health perspective. There's essentially no evidence that organic food is any "better" (or worse) for you, and the jury is still out for the effects of DHA: while there are some studies indicating it might slow the progression of Alzheimer's disease or inhibit growth of colon cancer, neither is likely to be a significant concern for the average two-year-old.
Safety is best thought of as tradeoffs, not absolutes. For example, given that motor vehicle accidents are the leading cause of death in young children, that $300/year would likely provide a far higher benefit if invested in (say) a better child seat or a reversing camera for your car.
Update for clarity: I understand the argument that there could be benefits to eating organic food and that, all things being equal, you probably don't want your kids eating any more pesticides, hormones, etc than necessary. My point is simply that there is currently no actual evidence that eating organic food makes any sort of perceptible difference, and you need to weigh the actual extra cost against both the possible benefits and alternative uses of that money.