Praise be to Allaah.
There is nothing wrong with taking assistance given by the government, whether it is Muslim or non-Muslim, to citizens, residents or refugees in its country, so long as the conditions for receiving this assistance are met by those who are taking it.
The one who takes this money is taking it lawfully, because the citizens and residents in a country have rights over its government.
Based on that, what the state is giving to your husband's parents is halaal money - in sha Allah - and there is no reason to feel anxious as to whether it is permissible to take this money to help you in taking care of them and looking after them.
Shaykh Abd al-Azeez ibn Baaz (may Allah have mercy on him) said:
There is nothing wrong with accepting assistance from the non-Muslim state if that will not result in failing to do an obligatory duty or doing something that is forbidden. They do not have the right to take assistance except in the official manner stipulated by the state, and they do not have the right to tell lies in order to get it.
End quote from Fataawa al-Shaykh Ibn Baaz, 28/239
And Allah knows best.