I don't think we're told definitively, but from her description at the start it's very hard to see how they could have had children when they were living abroad. I suppose given the upper class propensity to send them to boarding school it's not impossible but it doesn't seem at all likely. I happen to have my copy right here as I haven't put it back on the shelf yet, so here goes. This is what she says she thought the first time they had tea in the library:
We should grow old here together, we should sit like this to our tea as old people, Maxim and I, with other dogs, the successors of these, and the library would wear the same musty smell that it did now. It would know a period of glorious shabbiness and wear when the boys were young - our boys - for I saw them sprawling on the sofa with muddy boots, bringing with them always a litter of rods, and cricket bats, great clasp-knives, bows-and-arrows.
On the table there, polished now and plain, an ugly case would stand containing butterflies and moths, and another one with birds' eggs, wrapped in cotton wool. 'Not all this junk in here', I would say, 'take them to the schoolroom, darlings,' and they would run off, shouting, calling to one another, but the little one staying behind, pottering on his own, quieter than the others.
What a vivid imagination this very young woman has! That little detail about the youngest boy being quieter than the others is just lovely.
Anyway, I've managed to find the one and only passage I noticed that suggests Maxim also expects to have children. It's very late on, after the inquest.
'We'll start again, once this thing is behind us. We can do it, you and I. It's not like being alone. The past can't hurt us if we are together. You'll have children too.'