All the "big" State art galleries/ museums (Louvre, Orsay, Rodin, Pompidou if it was open but it's closed for a refurb) are surprisingly affordable for families, because U18s are free.
Jardin des Plantes is free (and lovely), and from there you can wander along the Seine towards Notre Dame, perhaps passing Place Tino Rossi where everyone goes to dance tango in the evening.
Best view of the Eiffel Tower(and IMO better than actually going there!) is from the terrace close to Trocadero metro.
Breakfast from a bakery - pastries and coffee to takeaway (Cafe a emporter) can be cheap as chips. You can get decent sandwiches from bakeries for about 4-5 euros. Keep an eagle eye out for "formules", especially at lunchtime- the French love them. Basically special offer menus of various kinds- the "formule enfant" menus for kids are usually particularly good value.
I had similar this summer- grand Interrailing tour of Europe, all transport and accommodation booked and paid for, then had several jobs cancel on me a month before we left, so the anticipated money for spends suddenly wasn't there. Let's just say that we basically travelled from Lidl to Lidl across Europe! 😆 We also went to a LOT of kebab shops- thank God for the Turkish diaspora!