Thanks, DYKWIA and c-w-s!
I'm not travelling this time, so am trying to send good advice their way. Must confess I'd forgotten about T3 to T5 (duh). I was slightly nervous for them that scheduled arrival at T3 is 1140 and the GLA flight is at 1455, in case anything went wrong. However, it was such a long time until the next available seats on the 2050 that with tired small children I understood the logic of going for the better connection and appealing for flexibility if badly delayed. Does that ring any alarm bells? If the incoming flight was delayed they'd be able to do something at DXB to start sorting it out. It is good to know that Heathrow is so quiet.
Re changing to GLA, they've already changed flights once (long story) and it's all been quite complicated in varous ways, so I came to the conclusion that I'm most supportive by helping them move forward with the plan they have rather than keep pointing out there might be a better one. They may also have wanted to be in DXB for the least time. But I appreciate the suggestion and I'd hope Emirates might still do that if needed, especially if they missed that connection for any reason.
Thanks again.