Question

there are many children having no parents语法上正确吗？

Answer 1

这句话语法正确。
分析:本句话是there be结构，主语是many children。having no parents是动词ing形式短语作定语，修饰名词children。
补充:句中having no也可以换为介词without。即，
There are many children without parents.

Answer 2

语法上是正确的。having以后为定语从句，修饰children。前边的主系表要素都有。