In choosing guardians, the number one factor you should be considering is who is the best person to love and raise your child? But while that person might be great with children, he or she might not be great with money, so it might make sense to place that responsibility with someone else that is better suited.
Similarly, it can be prudent to separate the responsibilities. This can create some checks and balances and ensure that guardians do not abuse their responsibility by having too much control and access over the assets as well. Furthermore, there may be many times when the trustee might be making financial decisions for the benefit of the child that also benefits the guardian, so there is a conflict of interest.
For example, what if your guardian already has children and needs a larger vehicle or to add on to their home? What about family vacations or a new computer or sending your child to summer camp? While it can be argued that these transactions would be for the benefit of your child, the guardian would also be benefiting from them.
Thus, it may make sense to separate the responsibility of raising the children from the responsibility to manage their finances, or at the very least, consider naming a co-trustee to work alongside the guardian.