Generally speaking, children born ab roadto US Green Holders can be sponsored by their GC holder parent (father or mother) as long as the child has not crossed 21 years of age and is unmarried.

In this particular case, the child, if born abroad, will fall under F2A category and you can apply for a GC for him, immediately after his/her birth or at a later time.....and it will take around 2 years for to get his/her own GC status.

ALSO, if you sponsor your child and he/she become a GC holder, and then you become a US citizen, your child automatically can apply for citizenship because he/she is a minor child and is dependent on a US Citizen parent. There should be no hassle as long as you're able to provide proper documentations about the child's birth and the verify relationship between you and him/her.

Whether to have the child take birth in Nepal or US is a personal choice, but from risk-free and convenience perspective, if your choice is to have him/her apply for a US citizenship at a later time anyways, it is better to have the birth take place in the US.