‘If I were a beneficiary, once I reached 18 I'd be suing you for the money plus interest/returns.’
indeed, this is an entirely possible scenario.
Said children are within their legal rights when adult, or to ask another to act on their behalf, to challenge the trustees - as mentioned above - & in due course if so wish to force the sale of the house to recoup their assets.
It is unwise to tie up other people’s financial interests in the very roof over your head, as OP may have learned already with her Ex. She may repeat the same error of risk assessment again if she ties up her fiscal security with her as yet unknown mindset of the children when adult.
OP’s children are now very young, far too young to have an opinion about what is in their best or preferred interests. Many years are yet to come, including some fierce financial adjustments in the employment & housing markets. I would not like to gamble on the value of current housing stocks in 15 years time.
There is a reason why the Grandmother did not split all her estate equally between her own children, & a rationale why she segmented a good proportion early on for her grandchildren’s benefit in trust with independent trustees.
Why, OP, do you think your mother did that?
Being direct, I think that your mother had a reasonable concern that you are not that financially savvy & easily bluffed/blustered in these matters.
As said by many posters, you have need to separate your own mortgage issues into a more manageable & efficient vehicle by using an independent broker. Then you & your fellow trustee need to act in the children’s best interests to have the considerable sum invested efficiently for capital growth for the next 15 years according to the terms of the trust.
Additionally, you may need to consider such financial vehicles to be ring fenced so that the children’s father has no potential interest. One sniff of spare money & you may find he turns up like the proverbial bad penny. Indeed were your children’s money used by you to ease your current financial mess so making the house more affordable for you, the children’s father may bring pressure to bear on them to gain their assets when they reach the age to force you to sell. Such matters do occur.
Get good financial & legal advice for you & for the children.