Taking your query a bit at a time:
It's not necessary to repeat to but you can do so if you wish.
However, your sentence would read more easily if you omit the word is
How is it possible to be a mother and [to] see your child being killed in such a horrible way?
Other readers might comment that there are no pleasant ways to see your child being killed but that would have nothing to do with grammar.
The second part of your sentence needs to be separated from the first, not by a full stop but a semi-colon (;) if it's to follow on directly.
to see them beating your child's head to a wall until his brain pours out.
This assumes that you have already identified the people you refer to as them
The word child is neuter and while it's quite correct subsequently to use the masculine pronoun his, which identifies the child as a boy, others might prefer to speak of its head or their head. This is just a matter of preference.
The child's head would be beaten (or dashed) against the wall rather than to the wall.
There's nothing wrong with pours out.Even though brains are normally more like a pasta than a liquid, under the circumstances they might well pour out.