How do I indicate to give a monthly payout to a particular person only if he/she assumes as the guardian to my child(ren)?

Within the testamentary trust (commonly from the same testamentary trust providing for the children), add the person under Trust Beneficiary.

Select the following configurations.

Give:	Monthly $
On the following condition:	Only IF he/she assumes the role of a legal guardian to my children, but stop when he/she ceases to be the legal guardian to my children (e.g. when my youngest child reaches age 21). This is mean to be a token of my appreciation for his/her role (if assumed) as the legal guardian.