Seeing as your DS is 3 and sounds almost ready to walk by himself I would get a single and a sling. Early on while bub is a titch you can push your DS and carry bub and once bub is a bit older, then you can push bub and DS could walk by then? Bribery might work? My DD went from terrible at the shops, running off etc. to really great really quickly.
We got a strider plus 4 pram just for the fact we can put a second seat on. But then we dont want another child until DD is 3 but incase things dont go to plans we have that option.
I think a sling & single pram as well. We have a P&T, awesome pram, but my DD2 who is 2 rarely uses it as she likes to walk. I've only used it as a double pram about 4 times since DD3 was born.
I'd get the single as well. What about those skateboard type things that attach to the back of the pram, would he stand on one of them? No idea what they are like, have never used one.
My DS likes to take his toy trolley to the shops with us so he can go shopping too! Not sure if yours would just take off with it but it seems to have subdued our DS.
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