Hi Helen, just a few thoughts to try & help ....
For St Petersburg, the tour operator Red October seems to get very good reviews from all the people on this site who hae used them. They will organise your Vusa'a as well, which may be a cheaper option.
For Stockholm, P&O usually operate a shuttle bus into the centre of town, frmo where it is very easy to get arouns & see all the highlights on foot.
Tallinn is VERY hilly, with cobbled streets, so a pushchair/buggy may be awkward
For Bruges a shuttle will take you to the local town of Blankenberg, sgsin fine for walking, and there is a large beach at the far end of the pedestrianised high street from the shuttle drop-off/rail station.
In Helsinki, there will again be a shuttle into town, i would say your best bet with a toddler would be to walk from the drop off point (if it is still the same) down through the gardens to the waterfront and fish market - WATCH out for the trams though, they are frequent and quite rapid
Finally in Copenhagen, there is no shuttle as the local buses run past the ship, however if you don't want to wander too far, then you can turn left off the ship, past the large statue of standing bears (oh plus a very busty mermaid) and follow the small marina around to the far side. Just beyond that point is the Little Mermaid, and yes, she is little.
If you were to continu along the flat waterfront you would pass statues, a fortress, the Amalienborg Palace, and to the head of the Old Docks of Nyhavn.
Of course how much or little you will want to do with your toddler, only you can judge, but hopefuilly this has given you some ideas.
Last: P&O Oceana December 2016
Next: NCL Jade August 2017
Then: P&O Ventura December 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(4), Oriana(12), Ventura(2), Victoria(5)
QM2(2), QE2(9), QV(7), QE, Caronia(2),
Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey, CMV Magellan