Phew! I felt relief; I had followed the mummy rules and got my child to share.
So imagine my annoyance when the other mother in this little unfolding drama failed to follow the rule in turn.
Having allowed her son to take my son's train, she then completely failed to encourage him to share it back after a while - a direct infringement of the rules!
OK, so it wasn't exactly a crime, but all mothers know about the mummy rules...don't they?
Mummy commandment 1: Thou shalt get thy child to share
If you get a really polite pair of mums, you can end up with toddlers passing a toy between them potentially ad infinitum. It's a crucial element of the rules that it makes no difference if your child refuses to share; that's what toddlers often do, and it's totally irrelevant.
You still make them share and then share back, and then share back again, because they need to learn good manners.
Mummy commandment 2: Thou shalt not say anything negative about another mother's young child
This rule only applies to infants; not children of six or over. No matter how badly behaved a small child appears, or how much they cry, it's very rude to acknowledge anything like this to the child's mother.
Comments like 'What's wrong with him?' or 'She's got quite a temper, hasn't she?' are against the rules.
Why? Because they imply the child is a problem, when the reality is that all small children are difficult sometimes and it's not a young child's fault if they are acting up.
Mummy commandment 3: Thou shalt not let thy child do something wrong without reacting
This applies both to letting your child actually hit or knock down another child, and also to letting them snatch from other children. These things do happen, but the key thing is that the eagle-eyed mother must spot such bad behaviour and respond appropriately.
It sounds so obvious, but this is one of the most broken rules, and often a cause of indignation.
Laughing misbehaviour off or refusing to acknowledge your child has been naughty can lead to serious mummy fights!
Mummy commandment 4: Thou shalt not give uncalled-for advice
'Helpful hints' like 'Don't you think your baby needs another layer?' or an accusing 'He looks hungry' said of a crying baby are hugely offensive to mothers who know perfectly well, thank you, that their baby has just eaten and hates too many layers.
These comments often come from strangers, but arise between friends, too.
For example, Chloe says, "If your baby has always slept brilliantly, then you probably don't have the key to solving someone else's baby's sleep problem.
And to suggest that you do is bloody annoying to a sleep-deprived parent who has tried EVERYTHING! Every child is different, and what works for yours might not work for another.
Mummy commandment 5: Thou shalt keep an eye on thy child
This follows on from rule 3. At a soft play centre recently I watched parents dump their children in the play area and then ignore them for half an hour while they had a cuppa. Meanwhile their kids were terrorising little ones.
Another day at a playgroup I saw a nanny neglect her two charges while chatting to her friend, leaving one toddler needing a nappy change badly, and both of them unable to join in the group activities. That sort of thing is just so sad.
Mummy commandment 6: Thou shalt not be smug
Whether it's boasting that you've got a perfect sleeper, or about your child's spot-on developmental milestones, or how they never cry, smugness is one of the biggest mummy crimes, and pride does seem to stem from naive ignorance.
"It's easy to be smug about what a 'good eater' your child is when they are 10 months old and happily chowing down on broccoli and asparagus," says Sarah. "From this perspective, you wonder in horror at parents who feed their toddler smiley potato faces five nights a week.
"Fast forward two years, and you will happily dish up whatever processed crap necessary to get your now much fussier child to eat a decent meal without tantrums, and the thought of him voluntarily consuming broccoli is but a distant dream-like memory."
Mummy commandment 7: Thou shalt make an effort socially
If you're a member of a group of mums who all meet at one another's houses, it's slightly rude not to volunteer to host yourself from time to time, even if a living room full of not just one but five screaming, stain-creating babies makes you shudder, and even if you're tired.
Similarly, when you're at a playgroup and someone sits next to you, it's mummy rules to smile and make chit-chat; there's nothing more irritating in the mum world than a frosty clique.
Mummy commandment 8: Thou shalt not be too gorgeous
OK, some women just can't help losing their baby tummy and being stunning, but the mummy rules definitely state that going out in a baby-sick covered top, with straggly hair, quite a few extra pounds and comfortable footwear, is absolutely fine.
After all, if you're looking after a small child, you don't exactly have time to spend glamming yourself up, and you might as well be proud of what motherhood has done to your body.
Mummy commandment 9: Thou shalt not publicly judge those who work or stay at home
Woe betide you if you break this one. Sandy says: "My little one is nearly one so the chats with other mums have been about going to back to work. We've decided that I won't go back and will be a stay-at-home mum. When I told the other mums this, one of them asked 'Won't you find it boring?' and suggested that I'm not achieving anything. It really upset me!"
Even if you feel strongly that mothers should stay home, or work, always at least act like you respect the other path.
Mummy commandment 10: Thou shalt give thyself a break
Sometimes, you just have to break the rules...go on, we're not watching.