One of the best gifts you can give your niece and nephew is teaching them some of the household skills; basic cleaning, laundry(sorting, temperatures, stain removal) and cooking (simple recipes).
It’s great to fill their freezer, but they will have to take this chore on someday. My friend was seven (and the oldest girl) when her Mom got appedicitis and peritonitis and was hospitalized for several months (this was back in the 60’s). Her step-father would not do anything that was considered “woman’s” work and so it all fell to Jan to handle.
She said that the day she had to take a pair of dirty underpants out of the hamper so that she could get dressed for school, was also the day that she took over in the house. She went to the neighbor next door when she got home from school and said “If you can show me how to do the laundry, I can do it from now on myself”. And she did. As a result, she is one of the most capable people I know and is not afraid to try any new skill. If these children have everything done for them, they will be totally overwhelmed and not know where to start.
You can be invaluable in teaching them skills they will use the rest of their lives. It will also bring the three of you closer together, these children will probably look to you to fill that gap, (as if anyone ever could). I’m so sorry for your loss, but these children are truly blessed to have you take such an active role in their lives!
BTW, my friend Jan became a little feisty at an early age. Her step Father was a lawyer and had a favorite white shirt that had to be carefully ironed. One day (remember she was just seven) she got fed up trying to iron this shirt to his standard and she just threw the shirt away. When he asked her where his shirt was, she reminded him that she was just seven and didn’t know that she was responsible for keeping track of it. What a gal!