I am so sorry for your loss. I believe at this point you need to do what is best for you and your family. It is not a time to worry about spending money on them or keeping kids happy with gifts. It is a time to be there for one another emotionally and physically. I think you should explain to your daughter that you understand how she was promised this gift, but things have changed due to the loss. Explain how money is needed to take care of funeral arrangements and etc. First think about if you desire to make this purchase in the future because if you do then you can express to her how you still wish to keep the promise her father made, but will take more time and you will try your best. Since you are now a single parent you are in control and need to do the best parenting you can do at this point. It is difficult when couples are on different pages when in comes to parenting. In anycase it is in your hands now, so you would need to do what you believe in for your family even if your late husband has done differently in the past.
I wish you all the best and please let me know if I can be of further help.