Is it permissible, Islamically, for a mother to transfer ownership of a portion of her gold to her minor children to avoid reaching the Nisab (minimum amount subject to Zakat) required for Zakat, with her intention to reclaim it later when they are older, and would this be considered a circumvention of Zakat?

If gold is for adornment, then according to the majority of scholars, zakat (definition) is not obligatory on it. However, according to the view that zakat is obligatory on jewelry, then zakat is not waived by gifting it to children if the gift was made for the purpose of escaping zakat. This is the correct view according to the Hanbalis and the majority of their companions. One should not resort to stratagems to evade a religious obligation, as zakat does not decrease wealth but rather increases it.