You can't. If you did not have actual physical custody of your child more than half the nights of the year, you can't claim the child and dependent care credit.
You are also not eligible for the Child Tax Credit, because you can't claim your child as a qualifying child dependent if they did not live with you more than half the year. You can claim them as an "other" dependent for the $500 credit, but only if you paid more than half the child's financial support.
The only person who can claim your child as a dependent is your grandfather, if he had custody more than half the nights of the year. Even though he is passed on, someone will have to file a 2021 tax return for him (usually the administrator or executor of his estate) for any income he received in 2021, and that tax return will include your child as a dependent.
There are some special rules for sharing the tax benefits of a child, but they only apply when the child is in the custody of parents who are divorced or separated and living apart. Those rules can't be used to allow you to claim the child because the person who claimed the child instead of you was a blood relative of the child (their great-grandparent.)
