nik76 - I'd do one single. It's easier to push around or park that way. They can easily take turns in one. We did three trips with a newborn and 3 year old in the mix and did just a single and a sling since we didn't want to bother with a double and it worked well. I realize you can't use a sling for that age, but 7 year olds should be old enough to take turns - or stop and take a break. Though my 7 year old doesn't think of strollers as for babies, she does not need one. She did sit in it on occasion on a trip when she was 5. Her 2.5 year old brother completely refused the stroller so I was stuck carrying him in the sling. (He has some sensory issues). So, I figured it was better that she ride than we push around an empty stroller. She didn't ride in it much, but enough that we were able to move around faster. When we went last time, she was 6.5 and she only sat in the stroller a few times, like when waiting when there was a ride she didn't want to go on. (We bribed the youngest, now 3.5, to sit in the stroller. The issue with him is that he wouldn't walk since he gets overwhelmed with the people around). Another way the stroller was helpful is that it has a strap attached. That way, the middle child could hold the strap while we walked so she would feel more secure about being with us. It helped when the eldest was 6 as well, but since he was more active and a wanderer. By having that strap, he stayed right with us.
Our eldest was over the weight limit of 50lbs of our stroller at 6 so he didn't get to even sit in it once that trip- and we did 7 different parks over 8 days. (He was only 3.5 on our trip before and walked most of the time at the start of the trip but used the stroller most of the time by the end of the trip). He normally complained about walking even short distances. We had hoped to get him in shape for walking, but it was a record cold winter here in the southeast so that didn't happen. (And we don't have appropriate clothing for that type of weather!) We told him that we would stay in each park for as long as the kids could handle it. If someone acted like it was time to go back, then we would leave. He rose to the occasion and made sure he was never the reason for us leaving. So, sometimes kids can really surprise you! We thought we would have to slow way, way down. I had the baby in the sling and the 3 year old in the stroller, so our pace was controlled by the 6 year old - but he walked like a champ!