I used to skate at Streatham ice rink, but wouldn't feel comfortable letting an 8 and 10 year old on the ice without adult supervision unless they were proficient skaters. Older skaters go round very fast and don't care if they barge into smaller people.
We go to the liverpool street one every year. It doesnt seem to get packed like the more fashionable one and some days there has only been a few people on it.. But i have never seen those seal things there.
Alexandra Palace rink has lots of seating and an ok cafe next to the rink. I'm not sure about their policy of letting kids on the ice with no parent but you could always ask. Every time in been in the past 20ish years they've had lots of available staff on the ice who seem very good a picking up any fallers and helping anyone who is struggling. You could then go for a nice walk through the grounds afterwards.
is there an ice rink in london that it will be ok to let dcs 8 and 10 go on alone ? i will have other dc (under 3) so wont be able to take them, The 10 year old has been before and is ok, 8 year old, i envisage will have a go with a seal aid thing and then give up but wouldnt be afraid to give it a go alone? They both want to go and i would like a christmasy outing with the 3 of them so dont want to leave the youngest at home.