الجواب وبالله التوفيق
In the situation inquired about, Shara’n there is no harm in using the child and disabled children support money given by the government for personal use as needed in addition to spending it on the children.
If the money is more than the needs of the children then this saved money will be considered امانت (trust) of the children, it was not correct to perform Umrah with the savings from the child and disabled children support money given by the government. Therefore, the amount you used for the Umrah from this savings will be counted as debt you owe to your child. You should pay this amount back gradually so your Umrah could become valid.
واللہ اعلم بالصواب