We have 5 businesses each with their own separate 401(k) plans.

There are 4 owners who have more than a 5% share in some of the businesses.

Owners include:
A is an owner in 3 of the 5.
B is an owner in all of the businesses.
C child of A, owner in 2 of the 5.
D is an owner in one of the businesses.

Businesses include:
1 is owned by A and B. A and C are participants in this plan.
2 is owned by B and C.
3 is owned by B (and others with less than 5% share)
4 is owned by A, B, and C. B and D are participants in this plan.
5 is owned by A, B, and D.

How would you handle the ADP/ACP testing in this situation? Would we have to aggregate all plans into one?

Sorry if I left something out, I'm new to this.

Before your question can be answered it needs to be determined which of the companies are in a Controlled Group or Affiliated Service Group as defined by the Internal Revenue Code. You have not provided enough information to determine either of those questions.