By way of general observation:
1. Many people, when making wills for small children, will not make any major distinction in their mind between trustees and guardians.
2. TO make the distinction clear, the trustees hold the assets and the guardians hold the children; they may or may not be one and the same person(s).
3. By way of expressing a personal preference, if I were guardian of any of my nephews\nieces, I would greatly prefer that somebody else have control of the money. I could then agree a monthly payment with the Trustee and\or payment on an as-needed basis. This has the advantage that when the kids were raised, they would not be in the awkward position of perhaps holding their foster parent to account for stewardship of the childrens' assets.