First off, I don’t have children, so you can take or leave anything I say as you please with no danger of offense. That said, I have been to Lourdes many times, and my observation is that there are so many places to stay that I don’t think you’ll have a difficult time finding somewhere that’s child friendly.
Here’s a link to the web page of a very nice place that you may want to have a look at, the Hotel d’Irlande. It’s very near the sanctuary, by the St. Joseph’s gate entrance to the sanctuary, so the wee ones wouldn’t have very far to walk. And it has a a very homey, comfortable atmosphere too.
Hope this helps. Good luck and have a wonderful pilgrimage!