Term for relationship to cousin's child [duplicate]

It is "cousin once removed".

See the nifty chart:
_{(chart credit: http://en.wikipedia.org/wiki/File:European_kinship_system_en.svg)}

To summarize, what you do is, you take whatever relation you are to the person in his direct ancestry of the same generation as you, and that's the "base" of the relationship; then, however many generations you had to move to get to 'your' generation, that's how far removed he is.

So, you want to know what relation your cousin's son (I'll call him "Fred") is to you. Obviously your cousin is the person in Fred's ancestry of the same generation as you, so you and Fred count as cousins of some sort. And, you only had to go one generation (from Fred to his parents) to get to the same generation level, so you and Fred are once removed from each other. Thus, cousins once removed.

Note that, as pointed out by AShelly, this process is only appropriate for figuring out people below you in the ancestral tree; the earlier-generation individual is the basis of the relation definition.

Well, you would call this person your cousin once removed, and they would call you the same thing, as per this Wikipedia chart.