Kiddyback tandem and a babyseat. Look for a used Dawes Toucan or Thorn for about £3-500. Will be the best bike you ever own. I added a tag along to mine once the baby grew up. You may struggle with train option but they are allowed and are bikes.

For the requirement (inc the train) a Tern Cargo Node or GSD would be just about perfect though.

You didn’t say which child was dropped off first. If it’s the small one, then you will struggle with the train without a tandem or separate bikes. If it’s the oldest, then a tag along and front mounted baby seat will work if you leave the tag along at the nursery. I used to leave out babyseat at the nursery then ride 6 miles to school and then onto work alone.

Tag along mount to a seatpost normally, but some will mount to a special rack. Burley is the one I used.

For new options, a Circe Helios would work and are pretty small for train use.

I used to do exactly what you’re planning. I have twins, and an older child.

I used a Yuba Mundo, with both kids seated on the back. Mixture of younger child in Hamax seat and older one sitting on the rear deck and holding on to stoker bars. When the eldest was old enough to ride, I’d have the twins on the back and he followed behind.

Hit this link for a pic of the Mundo.
The bike was pretty stable with a very wide kick stand. Although, it’s always a little tricky getting them loaded/unloaded without the bike toppling over.

https://www.dropbox.com/s/v06tlu6qk5n0b2e/2013-09-07%2009.43.30.jpg?dl=0

And on a useful note tag along bike you can remove and lock up at the station for the older child and the younger a child seat.

If you get hit by a car then it’s not going to be good for anyone.

Don’t disagree that coming off could be bad and certainly if you land on them then not good but I’m not sure a bike seat offers any protection in an accident.

That said I don’t plan to use my sling for bike use. I can recommend the Mac ride but that only does one kid.

I use a front loading cargo bike (like a bullitt) for my 3 yo twins, i also made a rear rack for it which extends out to the rear of the rear wheel so both of them can sit there if needed. (Google cycletruck caddyrack) i also have an old bmx saddle clamped to the top tube so in theory i could fit 5 toddlers on it, wouldnt like to try it though but i easily take both boys in the front box and my wife on the rear rack.

ut I’m not sure a bike seat offers any protection in an accident.

A rear mounted yepp style seat does offer limited protection in that it forms a abs plastic shell that absorbs impact. It’s of limited ability but it will take in some of the impact -and is far superior to dad landing on an unprotected child.

Jsut to be clear I said riding on a bike with a sling is a bad idea for all.

A trailer worked very well for me when dropping the kids at two different locations. I used to leave it chained up in the nursery car park, under a BBQ cover. That said I don’t know how easy it would be to take it on a train.

I’ve got a 5 yr old on the luggage deck of a Kona minute with an old stem/ cut down bars bolted to the seat post for her to hold on to and a set of stunt pegs bolted to the footpeg holes for her feet and the one yr old in a Thule one seat chariot. Great way to get around. those Tern elec assist GSD’s look the shizzle though. a real car alternative.

Slightly cheaper option that would be ideal

https://www.bikefix.co.uk/fr8

No propulsion from the 4yo though.

If you are putting the eldest on their bike and a tag along then look at the “follow me” I’m just headed off to work otherwise id write more and sort you a link but it’s great and can stay attached to your bike in a heavy but unobtrusive way and won’t interfere with a rear seat.

Like the look of the follow me but it’s quite pricey. And they don’t seem to come up secondhand much

Which is a surefire sign that resale value is high so you won’t lose much during ownership.

I had mine for about six years across two kids, the net cost after I sold it was about £50.