I split with exH 2 years ago. After which I moved back to Yorkshire from Scotland with our 2 DC, 4 & 8. He sees the kids EOW (which was the arrangement when still we lived nearby), drives down and picks them up from school on his Friday's at 3.45pm. Sometimes he's 20 minutes late or so because of traffic, and they pop the kids in after school club until he arrives. After school club runs until 5pm.
On Friday I got a call from the school at 5.10pm to say exH hadn't shown up and they couldn't get in touch with him. Luckily I live just 5 minutes from the school so I flew round to get them. They were both sitting in reception upset, especially my 4yo, at having no one to collect them and my 8yo was worried about her dad. Luckily by the time we got home he'd texted to say he was running late as there was snow on the A66 and he didn't ring as he had no signal on that road- and he was gonna be another 90minutes
I looked at BBC traffic, no reports about delays on the A66. I'm not friends with exH on Facebook but I did some investigating anyway as I was suspicious, and I noticed he had a close relative getting married in Gretna that day. I figured he was late because he went to the wedding or maybe for a drink after and lost track of time.
I decided I wasn't gonna pick a fight when he turned up, our drop offs and pick ups are very transactional and I want to keep it that way.
Anyway before he arrived the kids were asking where's daddy why is he late, why did he leave us at school. I didn't say he'd been at a wedding as I wasn't 100% sure, so I told them he said he had been stuck in traffic but he should have called the school and it was very bad of him not to.
So he turns up at nearly 7pm - 3 hours late! - with his brother in tow who's "come along for the ride"
both in shirts and smart trousers. His brother is tee total so I'm guessing exH had too many drinks to drive and so that's why his brother came along. Anyway I said nothing, sent the kids away.
Got a text late Friday night saying that the kids had moaned at him in the car, saying that mummy said he was naughty to have left them at school and not called. He had a good moan at me saying I don't have to slag him off to the kids I could have just stuck up for him and played it down with them, he couldn't call anyway because of no signal (which BTW is bullshit, the school said that his phone was ringing but went to voicemail). I thought fuck this, replied saying well it's a good job I didn't tell them then that he was actually at a wedding which is why he was late, and no I won't cover for his lies. I didn't want to spend my free weekend texting back and forth, and said as much, but he replied and is sticking to his stuck-in-traffic-with-no-signal story 
I'm setting off in a couple of hours to collect the kids and I have a feeling I'm gonna get moaned at for saying he should have rang the school or me. My stance is that it's not my job to lie for him to the kids, I'd much rather they be exposed to the fact that he makes mistakes and that he's accountable for his mistakes. I certainly apologise to them when I fuck up! I'm not going to pretend their dad is also immune from apologising.
Anyway WIBU to have not lied and said something like "Oh Daddy was stuck in traffic, these things can't be helped and he couldn't call the school"? The truth is he cocked up, lied about it, lied again and expects me to play along to this deceptive little dance.