Bike seats for 4 and 1y old - recomedatiosn please

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We are getting bikes and want to go out with ds on back - smallish 4y old and chunky 1y old. 4y old can ride a bike so will probably get a tag along type thing for him later but for now we would like 2 childseats - have hired bikes with them before and they love being on the back.

Mainly for forest trails, park etc and maybe the odd bit of riding up to the shops? (monutain bikes with road tyres)

Any recomendations and rough costs? The bike shop where we are getting the bikes will fit some for £39.99 - own brand I think. Was looking at the bobikes becasue they are cute but don't know if they are worth the cost?

Bumping for you.

DS, now 22 months LOVES the bobike mini - had him in it for the first time in a while on Monday (I'm 27 weeks pg, so a bit cautious about riding with him on busy London streets), and he adored it - keeps demanding his helmet and the bicycle. We used it from 10 months, I think. It's fantastic, I think - they can see what you're doing, where you're going; you can talk about what you can both see etc. Only minus is that if it's v cold/rainy they're acting as your windshield, so get REALLY chilly.

i have been looking at the bobikes but was thinking of a bobike maxi on the back of our bikes to start off with - I cant quite get my head round the front ones. the 1y old is quite tall and chunky and I am quite short so it looks like I would keep banging my chin on his head. Isnt it awkward to ride with your knees? Plus he can be a bit feisty and I worry he might throw his head back and smack my nose -he does this sometimes when I pick him up!!

Am thinking about 2 bobike maxis and then a tag along type bike for ds2 if I want to take them both out. Would this be doable on a mountain bike? I have quite good balance and am fairly confident on a bike.

Haven't tried the bobike maxi, the only thing I'd say from looking at other people's is that it doesn't have any sort of head support if they go to sleep. Which would be a big problem for me - I don't think you can/should ride with a really dangling head.
Can you try a mini? I don't bump my knees, the only bit that's at all difficult is getting onto the saddle sometimes (specially with bump!). My head is still well clear of DS's even with his helmet, but I do have a fairly upright bike and am quite tall...
I'm sure you can do two at once, my neighbour h as a mini and maxi on her bike and goes vertically uphill like that - amazing!

DS very happy in his Co-Pilot Taxi. I know it's not the cheapest on the market but it's really secure and has a grab-handle as well as straps. Rather depends how often and how far you plan to go, and what the roads/cycle paths are like.