@docmrm I also have an only child but have flown alone with her lots. The biggest help to me in the horrible toddler years was always remembering that it was a limited time and no matter how horrible it was it would be over soon.
Other than that, bring a stroller for the airport plus carrier for baby, especially if in Europe. You can gate check the stroller but may then have to walk up stairs to the plane or even get a bus, and after the flight it's only returned with the rest of the luggage. I know some people bring the small ones on board but I think it might take the place of hand luggage, plus you might also have to carry it up stairs. For the toddler one of those ride on suitcases might be useful.
Bring hand luggage with plenty of snacks (don't depend on plane food) and spare clothes for everyone, even you if possible. But check your main luggage, you have enough to juggle. Download some bluey or whatever your toddler likes to watch on a tablet. This is not a time to restrict screens.