Very young mammals have very little chance on survival when hand reared.
First of all they need specific food, secondly, they need a precise temperature, thirdly they need food round the clock at the times and in the amounts as their mothers would give it to them (and how much and at what times does depend on the species and possibly the temperatures and so on.)
And the further care, like skin care, massaging to improve bowel movements, keeping clean and whatever more are also specialist work.
For any animal where the parent(s) might still be around and able to care for the little one the best option is to leave it be.
Always a good option is to call a rescue center before you touch the young one, as often parents do not accept their own young if it has been handled by humans or if it has been away too long.
And by the time you found it, it may have been too hot or too cold, too long without food or care and not able to survive.
By taking in a very small mammal (or other wild animal) you set yourself up for disappointment.
(I have been able to see several motherless baby animals growing to adult, but in most cases ours were the only one out of the nests that were 'rescued' by good willing but wrong acting adults.)