worried about flying with two itineraries on the same day

Hi all-

Here's the short summary:

Family of four in DEN with youngest child under 12 (can not fly unaccompanied).

Grandparents live near EWR.

I'll be taking a vacation trip with the wife, and dropping off the kids (in EWR) on the way to our final destination of YHZ.

(DEN-EWR-YHZ) parents

(DEN-EWR) kids

On the return, the grandparents will take the kids to the airport and we will all fly back to DEN together.

(YHZ-EWR-DEN) parents

(EWR-DEN) kids

I booked tickets last night:

(4) Passengers DEN-EWR (reservation 1)
(2) Passengers EWR-YHZ (reservation 2) same day, 4+ hours after arrival in EWR

The return is basically the opposite:

(2) Passengers YHZ-EWR (reservation 2)
(4) Passengers EWR-DEN (reservation 1) same day, 2.5 hours after arrival in EWR

I don't think this violates any flight rules, but I had a few other questions.

Should I contact UA and let them know the plan and have them somehow tie the records together?
Can I check bags from DEN-YHZ for me/wife on outbound?
Can I check bags from YHZ-DEN for me/wife on return?
What happens if flight gets delayed from YHZ-EWR on return? Does UA consider us "connecting passengers"?
Can I have the grandparents check in the kids at EWR for the return, or should me/wife leave the secure area and checkin as a group of 4?

Have I created a potential mess that might leave us all stranded during IRROPS?

Anything else that I haven't thought of?

Any & all opinions are welcome. Thanks.

I have until tonight to cancel everything if necessary.