Oh no! You have a flat! You could
a) Quit running.
b) Bring your tire to a bike store and PAY someone to fix it.
c) Fix it yourself!
Step 0: Remove wheel from the stroller. The front wheel probably has a quick release lever and the back wheels probably have a quick release push button.
Step 1: Use a set of tire levers to remove the tire from the rim.
Step 2: Remove flat inner tube from tire.
Step 3: Carefully check the inside of the tire for any sharp objects that may have caused the flat. (You can also add air to the old inner tube and submerge it is soapy water to find the location of the hole.)
Step 4: Partially inflate new inner tube* before placing between tire and rim. This will decrease the chances of pinching it and causing another flat.
Step 5: Starting with the valve in the rim, slide new inner tube into tire.
Step 6: Lever tire back onto rim, being careful not to pinch the inner tube.
Step 7: Re-inflate to desired pressure (read the side of your tire for how many PSI are right for your tire. Ideally, use a hand pump with a meter. Try to avoid air compressors like the ones at gas stations as they are very strong and are likely to over-inflate your small tires.
* A note about inner tubes. If you can locate the hole on your old inner tube, you can patch it with a kit available at any local bike store. Sometimes it is just easier to buy a new tube. If you are not sure what size wheel (usually small = 12", medium = 16" and large = 20") you have, just pop it off and bring it with you to the store. It might even be the same size as your older child's bike wheels.
Thank you to my friend, Eileen, for taking the pics!