Thanks to everyone who responded with advice about Parque de la Costa. I thought I would repost and give our experience.
We live near the mitre train so we took that to Olivos and walked the four blocks to the tren de la costa station (Estacion Libertador). It was either do the walk between stations at the Tigre end or in the middle. It's not a long walk but one of the 5 year olds is not used to walking much and in the midday heat it was too much.
In general I would say the park is probably more fun for kids that are a little bit older and taller. There is a fair amount of stuff for smaller kids, and of course everyone's kids are different, but it's a lot of walking in the heat without much shade, and judging from the amount of time our kids spent in the little mini city, they would have been just as happy with Museo de los Ninos in Abasto . I'd wait until the kids are 1 meter 30. (Ours were under 1.10). Once we got them out of the mini city, there were a few disappointments over rides they couldn't go on, and one ride that they could go on, the airplanes, was so hot one of them got burned from trying to sit in the seat, cleverly made of black plastic. They couldn't even go on it, it was so hot to the touch.
It is really good advice to go during the week. We had originally thought to go on Saturday, when we could get an earlier start and go with the other parents, but decided to go Friday instead, even though it meant not having the other adults and leaving in the midday. The only thing that really saved the experience (so hot!) was the fact that there were hardly any lines, we maybe had to wait one ride cycle at the most. If you had to wait in long lines, it wouldn't be worth it at all with young kids.
Be careful in the pelotero--don't let the kids jump into the ball pit as it's touching the ground and not a soft landing (as it should be!).
The food is pretty bad and expensive, and they supposedly check your bags to make sure you aren't bringing food, but they weren't bothered by our water bottles (refillable) or homemade sandwiches, not that they looked very close. They're looking for two liter bottles of coke and more obviously packaged stuff I guess.
So an interesting adventure, like so many things here, but not one I'll repeat in the summer in the heat until my kid is a few years older. I vote for getting up early and hitting Parque de los Ninos and the juegos de agua! You'll want to get there at 10 and plan to leave by 1, as it's also crazy hot with not much shade, but younger kids especially will enjoy the water.
happy summer--anyone else have recommendations for fun things with kids, especially just turned five year olds?
