There's a tradeoff. To the extent you accumulate the income and gains in the trust, you'll probably pay income tax at a higher rate. The additional income tax cost may be more than it would have been in past years given the changes in the tax law that took effect this year.
However, to the extent you distribute the income (and to the extent you distribute the capital gains if you can do so in a way that they'll be taxable to the recipients), you lose the protection of the credit shelter (bypass) trust. The amounts distributed will be included in the recipients' estates, and will be exposed to the recipients' creditors and spouses.
The estate tax in the beneficiaries' estates won't be an issue as often as it was before, since the Federal exempt amount has been made permanent at $5.25 million (indexed), and portability has been made permanent. However, it will sometimes be a factor. Note that about 1/2 of the states have a state estate or inheritance tax, some with an exempt amount lower than the Federal exempt amount.
To the extent you make distributions to the children, the amounts distributed won't be included in the spouse's estate, but will no longer be available for the spouse if he/she ever needs the money. Similarly, to the extent you make distributions to the grandchildren, the amounts distributed won't be included in either the spouse's estate or the children's estates, but will not longer be available for the spouse or the children if they ever need the money.
The trustees have to consider these factors, and any other factors they think appropriate, in deciding how much (if anything) to distribute, and to whom.