They should be issued Forms W-2 and avoid SE taxes on the two younger ones.
Payments
for the services of a child under age 18 who works for his or her parent in a
trade or business aren't subject to social security and Medicare taxes if the
trade or business is a sole proprietorship or a partnership in which each
partner is a parent of the child. If these payments are for work other than in
a trade or business, such as domestic work in the parent's private home,
they’re not subject to social security and Medicare taxes until the child
reaches age 21. However, see Covered services of a child or spouse,
later. Payments for the services of a child under age 21 who works for his or
her parent, whether or not in a trade or business, aren't subject to FUTA tax.
Payments for the services of a child of any age who works for his or her parent
are generally subject to income tax withholding unless the payments are for
domestic work in the parent's home, or unless the payments are for work other
than in a trade or business and are less than $50 in the quarter or the child
isn't regularly employed to do such work
<https://taxmap.irs.gov/taxmap/pubs/p15-002.htm>
