Praise be to Allah.
The allowance that you received from the state is based on certain conditions, which you must adhere to, and it is not permissible to use tricks with regard to these conditions, because that comes under the heading of cheating, deceit and consuming wealth unlawfully.
If this allowance is only given to widows or divorced women, and if you get married it will stop, then it is not permissible to use tricks by not registering the marriage , because that will lead to many negative consequences, the most serious of which are:
1.. Consuming public wealth unlawfully. The state may take care of widows, but it cannot afford to take care of married women, and if a married woman takes public funds that she is not entitled to, then this is haraam wealth that she is taking for herself and her children.
Allah, may He be exalted, says (interpretation of the meaning): “O you who have believed, do not consume one another's wealth unjustly but only [in lawful] business by mutual consent” [an-Nisa’ 4:29].
The Prophet (blessings and peace of Allah be upon him) said: “The Muslims are bound by their conditions.” Narrated by Abu Dawood (3594); classed as saheeh by al-Albaani in Saheeh Abi Dawood.
And he (blessings and peace of Allah be upon him) said: “Schemes and deceit lead to the Fire.” Narrated by al-Bayhaqi in Shu‘ab al-Eemaan; classed as saheeh by al-Albaani in Saheeh al-Jaami‘, no. 6725. It was also narrated by al-Bukhaari in a mu‘allaq report in his Saheeh as follows: “Deceit leads to the Fire; whoever does an action that is not in accordance with this matter of ours, it will be rejected.”
2.. Not registering the marriage may lead to the loss of the woman’s rights, her husband’s rights, and the rights of any children that she has from him. If he dies, she will not inherit from him, and vice versa. If she has a child from him, she will not be able to register the child’s birth and have him admitted to school, and so on.
Moreover, at a time when dishonesty is widespread, the husband may develop an aversion to his wife, and then leave her and not acknowledge her, and he may refuse to divorce her, so she is left suspended. Or the woman may do that; she may reject her husband and deny that she was ever married to him.
The point is that what she wants to do of using this trick to take this allowance is not permissible for her.
What you mentioned about your husband’s income not being sufficient for your children is not an excuse to feed them with haraam wealth.
You must fear Allah, may He be exalted, and fear His punishment. You can either be patient with not being married, and continue to receive the allowance for you and your children, or you can get married – which is what we advise you to do, and strongly encourage you to do – and be frugal in spending on your daughters, so that their [step-]father’s income will be sufficient for them, and you can help them as much as you are able, even by doing some work from home. And we ask Allah to make things easy for you and bestow His bounty upon you.
And Allah knows best.