Luckily, you still have the aggron parent. That makes things immensely easier, not only because the aggron will do much of the work to take care of its child for you but also because aron will generally need many of the same things as aggron, just in smaller doses. Aron still need cool, dry places to sleep; rocks to supplement its diet (if you don’t feed it exclusively stone); and plenty of time to battle so it may gain experience and harden its shell. I would recommend a diet of pure stone, incidentally, as well as at least one hour of defensive training a day. Aron’s best traits on the battlefield is its defensive capabilities, so the harder its armor when it begins its time as your daughter’s starter, the better. It may also be worth it to invest in a sand bath of its own, complete with a few stumps or stones for it to Tackle. That way, it can bathe, play, and continue to work on training its defenses, all in one.
The only trouble you may encounter would be at the start of your daughter’s journey. Your late wife’s aggron may cause trouble, as losing her trainer would have been traumatic enough, but with the departure of her child and one of her caretakers, she may see the journey as less an opportunity for the both of them and more the act of losing half its family. It’s important to spend time with her and make it clear that you won’t leave her. If you have any other pokémon, have them bond with her too. If you don’t, consider getting her a long-term mate or ask the breeder who took on most of her clutch if she can visit until the hatchlings are adopted (or if she can spend time on the ranch, caring for any other pokémon—even this interaction will do wonders for her).
As for your daughter and her aron, have those two bond as much as possible. It sounds like the aron will still be young when it leaves home with its trainer, so you’ll need to focus on making the separation from its parent go as smoothly and as non-traumatic as possible. By having your daughter take an active role in caring for it—by feeding it, battling with it, playing with it, and so forth—the aron will bond with her more readily and, therefore, see her as its main family and caretaker, or someone who will be there for it and who it can trust.
In short, take care of the aron the same way as you would an aggron. The same could be said after your daughter begins her journey. She’ll need to continue feeding her aron and battling with it regularly. The only change is that she may need to polish her aron by hand, using a rag and standard steel-type polish, available at any pokémart. It’s just that prior to her departure, ensure that both your wife’s aggron and your daughter’s aron are emotionally prepared for the separation.
Good luck, and congratulations on your daughter’s journey!