Let $A$ be a child has a stuffed dog. Let $B$ be a student has a stuffed bear.
You know $Pr(A)$, $Pr(B)$ and $Pr(\bar{A}\bar{B})$.
If you sketch out a Venn diagram, the following relationship will be easier to see.
$$1 = Pr(A) + Pr(B) - Pr(AB) + Pr(\bar{A}\bar{B})$$
So after some rearranging we have,
$Pr(AB) = -1 + Pr(\bar{A}\bar{B}) + Pr(B) + Pr(A) = -1 +1.15 = 0.15$.
The points made by others above are completely true, but I think keeping the scope of the problem in mind, you'd be safe to assume that $Pr(A \cup B) = 0.85$.