Minors are beneficiaries of an overseas (UK) trust. Tax implications?

I am a US citizen, and I have 2 daughters under age 10 (also US citizens). My parents are UK citizens and live abroad. They are setting up a trust which allows them to gift a certain amount each year to chosen beneficiaries. They are looking into gifting my 2 daughters a certain amount (around $15k) annually, but we are unsure of the tax implications.
 
Would this count as a gift, and thus untaxed? Or would it count as earned/unearned income, and thus potentially have a tax burden?