This is probably the most useful page this week. The goal is to let the kids actually do something rather than drag them through places in 38°C heat.
This is the closest match I found to the “really cool kayak about an hour away” your kids mentioned. I cannot prove it is the same school/bus outing, but it fits the description very well.
The club’s self-hire route is 5.4 km and takes about 1½ hours, with no stated difficulty or disembarkation. Current summer rental hours are 13:30–18:30, last 1-hour hire at 17:30.
A serious aquatic centre rather than a small municipal pool: 50 m Olympic pool plus leisure water, slides/toboggan and river-style features. This is the safer bet if the children mainly want to get wet.
From nightfall, the cathedral and other monuments become projection canvases. The 2026 season runs 11 April 2026–3 January 2027. It is a very easy “special ending” to a Chartres water outing.
The municipal description says the café-vélo offers regular and electric bike rental, repairs and charging, and is designed for families as well as cyclists. The missing piece: I found no published child-bike sizes.
Worth a phone call; if they have four suitable children’s bikes, this is the nicest “start near home” solution.
This is the most dependable rental plan. The official Petite Venise point rents bikes daily in summer, currently 10:00–18:45 (last departure 18:15). Rowboats on the Grand Canal are also available daily in July/August.
Child sizes are not specified on the published page, so ask at the rental point before designing the whole visit around bikes.
Paris has free summer swimming at several sites, including Bassin de la Villette and designated Seine/Canal zones. For river sites, openings can change with water quality and weather, so this is a same-day decision rather than something to promise the children.
La Villette is the simplest family-style option. Bras Marie is the more extraordinary “we swam in the Seine” memory; children under 1.20 m are not admitted there, and under-14s must be accompanied.