Thought I would do a small calculation:
I found that the safe tolerable limit of arsenic is 2.14 µg/kg/day. So for a 10kg toddler (which is on the small side) the maximum tolerable limit of arsenic is 21.4 µg a day.
So I looked up some values for arsenic in brown rice, foudn some here:
http://www.whfoods.com/genpage.php?tname=foodspice&dbid=128#safetyissuesSo to be on the safe side, let's assume the worst case scenario: US rice with 100% inorganic arsenic. So assume 0.4 μg/g of inorganic arsenic (bear in mind that in reality the amount will be lower than this.)
I make my rice/quinoa milk with 1/3 cup each of organic brown rice and quinoa and it lasts 3 weeks (frozen in batches) for making my coconut milk eggnog. 1/3 cup is about 50 g and so it contains 0.4*50=20 μg of arsenic, consumed over a 3 week period. That's around 1 μg a day! Less than 5% of the than maximum tolerable limit of arsenic which is 21.4 µg for a 10 kg toddler (and my toddler is 12 kg so his actual max limit is higher than that).
All in all, the egg-nog I make contributes less than 4% of the maximum tolerable intake for his weight so I am not going to worry about it.
Not sure how much arsenic is in the commercial stuff though, but I found this:
http://www.tastethedream.eu/safety/Obviously the levels would vary from drink to drink so it might be worthwhile checking. Or you could dilute the rice drink further with coconut milk.