From your description of your set up I would say no.
Our two live in a large converted stable, we have a large 4 bed home, only 1 DD left at home (5) she is still at the right/wrong height for a face full of snot every time the girls move.
Do not under estimate the amount of slober these gorgeous dogs produce, or the distance it can fly when they shake their heads. This is the reason ours have a stable, every conceivable surface was constantly covered.
Whilst these dogs are gentle giants, I would not entertain having one in the house with an 18mo. Having 15 stone of dog brush past would knock them flying.
An average size garden will not cope with the mini roundabout size turds produced, how would you keep children/dog poo area separate while still leaving enough room for both to play.
These dogs are immensely strong, how, as a single parent, would you plan to walk the dog? That's not meant to sound snippy, but there is no way I can walk ours with Dd in tow.
Vets fees are constant and huge, sadly this breed is prone to many health issues hip and elbow displasia is common, as are convulsions. The last time one of my girls was ill it cost of £7k for one weeks treatment.
They are gorgeous and loyal pets, BUT, they require a massive amount of effort and dedication.
Sorry to sound so negative, but it's obvious from the amount of Bernards in rescue centres, how many people soon realise they don't have the room or time for them.