Well, first of all it is better to book additional ticket on the name of the passenger who is actually traveling, so in your case you (or your child*). That way if some issue would arise you would have a solid position, because even in the
RZD rules (it's question-answer, but in the answer RZD official gives a reference to the particular article regulating the issue) it's officially written that passengers can buy out the whole compartment on the bases of full ticket issued on the name of the one of the passengers traveling, no problem that two tickets will be issued on the name of the same passenger.
*Regarding child ticket it's a bit more tricky. Obviously buying child ticket is cheaper, however than you basically don't follow the rules, thus if for some reason carriage conductor will decide to accommodate some outsider in your compartment (usually passenger without a ticket who "bribes" the conductor for a short ride, in Russia it still happens sometimes), then you will end up in a rather delicate situation - from one side conductor in such cases operates on illegal grounds, but on the other side you can't really present your additional child ticket (in order to prove that the bed in your compartment can't be taken by anybody) because you also in a way bypass the rules and purchased cheaper child ticket instead of full fare adult one. Although you also might get lucky and travel the whole route without a need to show anybody your fourth "child" ticket. So at the end child ticket is a bit of a risk, justified or not - it's already your choice. Besides the above issue with child tickets we also heard that some passengers had trouble booking two child tickets on one name/document, RZD website simply didn't allow that. Not sure if it's still an issue, but it might be and then you basically left with the only choice of getting full fare ticket.