Good question. I ride a triplet and have had my little one in the middle with a child stoker kit since 4 years and 1 month. I nearly always put her on while the bike is leaning on something (Usually either the tree out front of the house or the side of a building). Of course, this is not always an option if, for example, changing a flat on the side of the road. In these cases I step over the captains tube facing backwards, straddle the bike and lift the little one on. Then I step out and back on the right way.
I find that when riding in groups, many people offer to help, but few of them understand that the most important part is the front wheel. They usually hold the seat which usually results in the front wheel turning and me having to catch the bike before it falls. So I usually tell them I've got it under control.
She is now beginning to climb up by herself (turned 5 in September) so I have to make sure I'm holding on to the bike well and I have to be very patient because it takes her a couple minutes. Good luck. It is so much much. The little one and I went on a 20 miler in the sub 30 degree weather to look at Christmas lights. She was so distracted (and covered in layers of clothing) that she didn't even complain about the cold.