A lot of people I know (yes, you!) use their children as an excuse for not doing physical activity. They don’t have anybody to watch them, they tell me that they  don’t want to “waste” time on themselves and would rather do something with their children. I instead have always tried to incorporate my children into my sport activities or me into theirs.
Before I had my two children I used to wonder how I was ever going to be able to keep running, swimming, biking once I had them. I don’t have any nearby relatives that I can rely on and my husband’s mother lives 700 kilometres away. On top of that my husband worked four days per week out of town and on those days I was completely on my own. If anybody had an excuse to not stay active, that was me! I really had find a way for it to work without much outside support. I needed to pull in all my creative energy and think of new ways to workout while I still pursued my running or triathlon goals.
When they were newborns I would either go for a walk with them in a baby sling or wait until they were napping to do some “running”. I called it running but it was more “moving as much as I could in different ways”. I would set up running circuits through my house and out the back yard and then back by the baby crib to see if the baby was still sleeping. That would happen about every minute!  Sometimes I would splurge on a babysitter or exchange time with another mother. My longer runs were all done when my husband was able to take over and stay with the kids.
Once they were older able to sit up  it was a lot easier because I had my baby jogger. This was no problem for my daughter, she just LOVED that stroller. I trained for a marathon with her in it since she didn’t mind going for  two hour runs in it. My son, ten years later, was a whole different story. I could time my runs with him because once we hit 30 minutes he wanted OUT of that thing! No marathons for me that year…
As the years went on and they got older I came up with different ways to incorporate them into my running workouts: bringing them to the park and running around the jungle gym, playing ball with them on a field and running like crazy when I had to fetch it; setting up exercise circuits that they could pretend to do  with me.
In the later years I had a huge breakthrough with my running when my son finally learned to ride a bike. When spring came and the weather was warmer he  started  following me on some of my runs. After just a few months I actually had to follow him. In the end we got up to 10k bike/runs and I have to say that now those are some of my most precious memories with him as a young child. 
He would chat mindlessly to me about all sorts of topics and if it he wasn’t riding too fast, I’d try and respond between gasping for air!
In the end, you can run and spend quality time with your children at the same time. No excuses necessary.