Can we recite ayat ul-Kursi on an infant?

Yes. It has not been prescribed for a special reason, but you can - for those who can. It is just like the three Quls for example. You know your kids cannot do it. So people have commonly recited themselves and then wiped it on kids until they are able to do it themselves.[1]

Reference:

[1] http://islamqa.info/en/6092