@Rachel I think my FIL may be bringing his, if not, we have a wagon with a snap on roof! We also plan on bringing stroller fans for them!
How about a little baby pool (with supervision of course) with a beach umbrella over it?
@Sarah I love that idea! I just don’t know if we’ll have access to water since it will be at a park!
@Fabiann take bottled water with you?
Most parks have spigots you could take your own hose if you have one
Add ice cubes to your water cups and consider bringing a big thermos of just ice cubes to use throughout the day. Keep milk bottles in a cooler with ice when not in use. Use their travel lids and/or a plastic bag if you need to use someone else's cooler. You can pre-freeze snacks (like berries, cut grapes, and peas) so they will stay cool longer and be safe for your 1yo to eat when they thaw out a little. Glad your BIL is cancer free!
Can you put up a little tent with some toys in for them to play in?