If there was no condition stipulated by your husband that you may work provided you spend all or part of your salary on household expenses, then the salary that you get or the amount that is left after you pay the amount that your husband stipulated you should pay, is your property and it is permissible for you to spend it in any way that is halaal, and you do not have to ask your husband’s permission with regard to that. But if you do that so as show respect to him, that is better.
Helping one’s parents is one of the greatest acts of kindness and goodness, and it comes under the heading of upholding ties of kinship that increases one’s lifespan and brings blessing in one's provision, as the Prophet (peace and blessings of Allaah be upon him) said: “Whoever would like his rizq (provision) to be increased and his life to be extended, should uphold the ties of kinship.” Narrated by al-Bukhaari (5986) and Muslim (2557).
Based on that, there is nothing wrong with what you are doing by helping your parents with part of your salary without your husband’s knowledge, especially since you say that he does not mind, but if you think that telling him will make him feel respected, then that is better.
We ask Allaah to reward you for this help and to bless your wealth and your work for you.
And Allaah knows best.